Tl;dr: Bringing Intelligence up to 20 is necessary at some point, but doesn't have to be immediately. War Caster and Resilient are both great options and will help you maintain concentration. Heavy Armour Master and Lucky are also great options but won't have much impact on concentration. None of the other feats seem particularly worthwhile for you. Personally, I'd probably take War Caster and then decide between Heavy Armour Master and bringing my Int up to 20.
I'll address your points one by one, starting with:
Bringing Intelligence up to 20
You definitely want to bring your Int to max at some point, because it controls your primary means of attack. The question is whether you want to get it now, or leave it till later. With that in mind, consider what 2 more points of Int actually provide:
- +1 to hit with your spell attack
- +1 to DCs for your spell saves
- +1 to Int saves
- +1 to Int checks
- +1 to number of spells you can prepare
None of these are huge benefits in the short-term. Long-term, this makes you 5% more effective at most of the things you specialise in. (Warning: gross over-generalisation.)
War Caster
You don't intend to have both hands full, so this only gives you advantage on Con saving throws to maintain concentration, and spells for opportunity attacks. However, both of these are strong benefits. Advantage to maintain concentration needs no explanation, but spell opportunity attacks might not be as clear.
To start with, you're planning to be in the thick of the melee, so you should be getting opportunity attacks often enough to want to use them. Unfortunately, you aren't planning on using a weapon, and your signature spell Vampiric Touch won't work with opportunity attacks. (It specifically requires an action on your turn to make an attack with it.) So in your present state, you have no use for opportunity attacks. Giving yourself an option could pay off.
On the other hand, opportunity attacks aren't the only reaction, and you might want to save your reaction for spells like Shield or Counterspell. That said, these require resources, whereas with War Caster you can always use your opportunity attack to cast a cantrip.
Heavy Armour Master
As you say, the Strength is negligible where you're concerned, so the question is the damage reduction. This is a great option for someone planning to be in melee - 3 damage off every non-magical weapon attack adds up to be a lot of damage very quickly.
For concentration, it's a bit weird. It won't affect the vast majority of Con saving throws you make to maintain concentration, since the damage has to be at least 22 for it to have an impact. (Many high-damage attacks are magical, and it won't affect those either.) When it does affect the DC, it will only reduce it by 1-2. So as an option to help you maintain concentration it's a bit of a bust. It's still a great defensive option though.
Keen Mind
I can't say any of these features have ever come up in a game I've played. They're all highly situational and I can't find anything to redeem this feat other than "it's kind of cool, I guess".
Linguist
3 languages is pretty good, but there are spells that take care of this sort of thing for you. I've never seen a situation in game where a cipher would've been handy. If you're looking for a way to send messages to someone that no one can intercept, there are a variety of spells to achieve that.
Lucky
3 rerolls a day that you can choose to spend once you've seen the result? This is huge. You can spend them on an attacker's roll against you? That is just nuts. If you get critted, you can just wipe it away. Note that its benefits aren't nearly as potent on rolls that you make with advantage, but they're even more helpful on rolls you make with disadvantage. As far as concentration is concerned, you can fix a failed concentration roll 3 times a day, but you're likely to be making a lot of them, so something that isn't a scare resource and benefits all of them is probably worth getting.
Observant
This feat is frighteningly good. Just not for you. For a character who's maxing Wisdom and has proficiency in Perception, this makes them a portable radar station. (One of my players, the cleric, took Observant and has been making my life hell spotting every stealthed enemy and trap a mile away.) For you, it will make you almost as good as they were without it. If your party lacks a "sensor" character, it could be worth considering. Assuming you have one, I don't recommend this.
Everything in the above paragraph is based on Perception, without considering Investigation. That's because Perception is used far more often than Investigation, and passive Perception is used almost constantly whereas passive Investigation rarely sees the light of day. This can vary from campaign to campaign, obviously, but in general Perception is better.
Lip-reading is a cool little feature that I have seen used several times in intrigue situations, but I don't think it's worth spending a feat on.
Resilient
For maintaining concentration, this is pretty strong. More reliable than the advantage provided by War Caster, and scales better as you level. As well as that, though, there are a lot of effects that trigger Con saving throws. Most diseases, poisons, many spells, and a lot of monster effects. There's a saving throw for every stat, but Con, Dex, and Wis are still the most common, so this is definitely worth your consideration.
Ritual Caster
First and foremost, you have every benefit of Ritual Caster (as far as spells on the Wizard list are concerned.) Second, the benefits of this particular feat are highly dependent on what scrolls the DM feels like tossing your way. So the only quantifiable benefit of this spell for you is learning 2 1st-level spells from another class's spell list. Unless you can see something you really want (I can't), I can't recommend this.
Besides that, most rituals are either available to the Wizard or can be effectively duplicated by the Wizard. The only one I can see that you can't produce for yourself (barring a Wish, obviously), is Silence, which I can't quite see how you would use as a ritual. (It's more of a combat spell, and the 10-minute casting time is inconvenient in combat.)
Wizards study, Sorcerers know
Wizards practice learned magic. They study for years to learn how to manipulate the weave and become masters of it.
Sorcerers practice innate magic. They are gifted with an ability to manipulate the weave, and sculpt their interactions with it.
Class features
Wizards use Intelligence as a spellcasting modifier, while sorcerers use Charisma. These abilities are associated with different skills, so that difference may or may not be important to a player.
Wizards know more spells, and prepare a subset of those spells to cast each day. A wizard learns 2 spells per level, and starts with 6 at level 1, totalling 44 spells known. They prepare a number (INT+Level) of spells from that list each day to be able to cast. Wizards can also copy spells from spellbooks and scrolls they encounter, adding those to their spells known.
Sorcerers know fewer spells, but can always cast them. Sorcerers learn a total of 15 spells, but they're always able to cast them if they have spell slots available.
Wizards can cast more spells per day through their Arcane Recovery class feature. They restore a number of spell slots when they take a short rest.
Sorcerers can sculpt their spells to behave differently. Sorcerers have access to Metamagic, which allows them to increase the damage, range, or duration of spells, grant disadvantage on saving throws against them, cast silently, or affect an additional target.
Sorcerers learn more cantrips than wizards.
Wizards can cast spells as rituals, taking 10 additional minutes to cast, but without using a spell slot.
Best Answer
You will mostly be missing jokes referencing events and characters.
The majority of the missing context will be jokes and inconsequential references to old characters and historical events in the world's timeline. This was the case in Campaign 2 (and has so far been the case in Campaign 3); occasionally one of the cast would crack a joke about how a Vox Machina character would have acted in a particular situation or something like that, or one of them would remember something and Matt would remind them that was something that happened to their Vox Machina character, not their Mighty Nein character.
Matt is very intentional about not relying on past characters too much.
In the first episode of 4-Sided Dive, the successor to talk show Talks Machina, Matt gives some commentary into how he has approached integrating characters from past campaigns into Campaign 3. This episode aired after C3E18, so it contains spoilers up to that point, as well as spoilers for ExU: Kymal. Here are some relevant quotes that give some insight into this question:
So it seems Matt is being very intentional about keeping the narrative of Campaign 3 largely self-contained, even if some character backgrounds do involve characters from previous campaigns.
It may be worthwhile to catch up on Exandria Unlimited as you follow Campaign 3.
Exandria Unlimited is an immediate prequel to Campaign 3, and (minor spoiler):
Further, we learn (minor spoiler for Exandria Unlimited):
Overall, I think you should be just fine to start with Campaign 3, especially if you catch up on Exandria Unlimited as time permits (8 episodes, roughly 32 hours of content). If you don't intend to actually sit through all of Campaign 1 and 2, you can just follow up on the lore threads from those campaigns as you encounter references you are unfamiliar with in Campaign 3.
The Critical Role website has a helpful article, "What you need to know before watching Exandria Unlimited", that suggest that Exandria Unlimited is a good place to start for those new to the series:
So you definitely don't need anything from Campaigns 1 and 2 prior to Campaign 3, but Exandria Unlimited may be helpful for providing some of the context for the beginning of Campaign 3.
For a broad overview of the history of Exandria, watch this video: Exandria: An Intimate History (Campaign 1 spoilers). Replays of Exandria Unlimited can be found in several places:
Update through Episode 6, spoiler block contains spoilers
We learn in episode 6 that a character's backstory relates heavily to a certain antagonist from Campaign 1. Watching Legend of Vox Machina on Amazon Prime should get you up to speed without having to watch all the live play episodes from that part of Campaign 1.