So I think that I'm seeing two real questions here:
- Do on-hit effects proc spell vamp?, and
- Can items proc spell vamp?
The first question is a bit of an odd case, because it really depends on what you mean.
On-Hit Abilities
Some abilities, such as Gangplank's or Ezreal's Q are treated exactly like auto-attacks. These trigger on-hit items like Black Cleaver, as well as trigger Life Steal and other effects (depending on the champion).
On-Next-Hit Abilities
The second kind of attacks are "on-next-hit", which modify the champion's next attack and reset the auto-attack timer. These are abilities like Nasus' Siphoning Strike, Blitzcrank's Power Fist, and Garen's Decisive Strike. These also trigger Life Steal and not Spell Vamp.
However, to confuse things, there are also "On-Next-Hit" Abilities that trigger spell vamp, such as Poppy's Devastating Blow, and Mordekaiser's Mace of Spades.
Note that Poppy only has spell vamp in this picture. The 15 damage that is visible is from Malady's proc, which was triggered by Devastating Blow.
The only difference between the two types of abilities is that one adds a bonus to your auto-attack, and the other one replaces your auto-attack. The easiest way to test this is to use the ability against a tower. Since towers are (essentially) immune to abilities, if you use it against a tower, and it does not consume the charge of the ability, it uses spell vamp. If you do manage to deal bonus damage to the tower, it uses life steal.
Physical Damage Abilities
Finally, you have some abilities that deal physical damage, but are not auto attacks. These are things like Garen's Judgement, or Urgot's Acid Hunter. These things deal physical damage, but, because they are abilities, use Spell Vamp instead of Life Steal.
In general, any given ability onlys benefit from Life Steal or Spell Vamp, not both.
So, should you rush a Will of the Ancients? It depends entirely on what champions you have on your team.
Spell Vamp and Items
As to your second question, the answer is much simpler: Item's with on-hit damage effects, such as Wit's End, Madred's Razor (and derivatives), or Malady, never proc Spell Vamp.
Deathfire Grasp or Hextech Gunblade, items with direct activations, however, will.
Bonus HP acts exactly like regular HP. It is subject to the same damage reduction/amplification from resists. In addition, it is added to both your current and maximum health, but the additional max HP is only temporary.
At the expiration of the spell, any bonus HP you received will stay. The exception to this is if your current HP would be greater than your regular, unbuffed max HP. If this is the case, your HP will be reduced to your max HP.
For example, if you're at 400/1500 HP and you get Wild Growth'd for 300 HP, you'll be at 700/1800. If you proceed not to take any damage, you will be at about 700/1500 at the end of the 7 seconds.
However, if you're at 1500/1500 HP and you get Wild Growth'd for 300 HP, you'll be at 1800/1800. Suppose you take 200 HP of damage. This time, when the spell expires, you'll lose your extra HP. The 1600/1800 HP you have just before the ult wears off will become 1500/1500, since you cannot have more current HP than maximum HP.
One thing to note, by the way, is that all the current sources of bonus HP actually increase your maximum health (temporarily), meaning that % max HP damage sources will do slightly more damage than if you had unbuffed health.
Best Answer
If you're going for pure AP, then obviously you want to have a Rabadon's Deathcap. It grants 120 AP, and has a unique passive that modifies your AP up 30%.
Another item you'll almost certainly want is Seraph's Embrace. It only grants 60 AP, but grants 1000 mana, and has a unique passive that grants an additional 3% mana as AP, this makes it worth more than a second Deathcap as soon as you have 1000 mana from other sources.
Lichbane is worth only 87.5 AP, not worth taking in a pure AP build
Rod of Ages is worth about 100 AP, still not worth taking in a pure AP build
Mejai's Soulstealer, fully stacked, is worth 180 AP, definitely worth taking if you can keep the stacks up. It drops to 124 on a single death. (13 stacks)
Deathfire Grasp is worth 120 AP, exactly as much as Rabadon's Deathcap, and would make a good filler item, as while the Active won't stack, you will get the 10% cooldown reduction repeatedly.
Zhonya's Hourglass is also worth 120 AP, and can be stacked for Armour.
Add in Elixer of Brilliance, and Exalted with Baron Nashor for an extra 80 AP and 10% CDR
Final Build: Boots, Rabadon's Deathcap, Seraph's Embrace, Deathfire Grasp, Zhonya's Hourglass, Mejai's Soulstealer. Elixer of Brilliance and Exalted with Baron Nashor. =(120+60+120+120+120+40+40+3%mana)*1.3 = (620 + 3% mana) * 1.3 = 884 + 3.9% mana.
This itemization is the best you can do with boots for a generic champion.
This should work for almost any given mana'd champion with any given rune/mastery set. Here is a link to this build on Anivia (my main) for 1166 AP
Edit: Sorry, I missed the part about 3v3, That section below:
3v3 Rabadon's Zhonya's, and Mejai's are all disabled, as are Exalted with Baron Nashor and Elixer of Brilliance.
Wooglet's Witchcap, while only 100 AP, multiplies AP by 1.25, and so is still worth taking.
Exalted with Baron Nashor is replaced by Crest of Crushing Wrath, also worth 40 AP, while Elixer of Brilliance is replaced by Ichor of Illumination, which yields 64 AP, up from 40.
Additionally, the 2 altar bonus from Twisted Treeline gives an extra 10% AP. In total the build consisted of 3 Deathfire Grasp's, Seraph's Embrace, Boots, and Wooglet's Witchcap, and I was able to get 1131 AP.