tl;dr -- You always use the highest absolute value for an ability or skill, then modify by relative values.
There are two types of modifiers: relative and absolute. Absolute modifiers do not stack, while relative modifiers may stack, depending on the particular RAW for a specific modifier.
Let's take a look at the absolute modifiers in play:
Gloom Sight (or goggles): This feat and item each grant your character exactly 90' of darksight. No more, no less. This is an absolute limit. The goggles even state that they have no effect on Gloom Sight.
Deepsight: This feat grants your character exactly 120' of darksight. No more, no less. This is an absolute limit.
Now, let's look at the relative modifiers:
Nightblade 3rd level: The RAW says you gain darkvision 30', or if you already have it, an increase of 30'. This is a relative modifier. It modifies whatever your absolute value is.
So, you can only use one of Gloom Sight, Gloom Sight Goggles, or Deepsight. There's no benefit to using two or more of them together, because the RAW specifically states that they set an absolute value. If you later gain a more powerful version of darksight (Nightblade's 14th level, for example), then Deepsight no longer grants a benefit either. The most powerful absolute value is the only value that matters. All lesser values are dispensed with.
However, you can still benefit from the relative modifiers. This means that at 3rd level Nightblade, you'll have 150' of darkvision with Deepsight, and at 9th level, you'll have 180' of darkvision. At 14th level, this all becomes pointless, because you're no longer limited by darkness at all. You can see things from miles away if the atmosphere allows it; darkness no longer has an effect on you, so it may as well be bright as day all the time from your perspective.
Interesting point regarding the description of darkvision on PHB p. 184. However, the Monster Manual on p. 9 says:
Darkvision
A monster with darkvision can see in the dark within a specific radius. The monster can see in dim light within the radius as if it were bright light, and in darkness as if it were dim light. The monster can't discern color in
darkness, only shades of gray. Many creatures that live underground have this special sense.
This definition matches the PHB definition on p. 20. Comparing the PHB p. 20 and MM p. 9 definitions on the one hand with the PHB p. 184 definitions on the other, the only part in discrepancy is the part about "see in dim light within the radius as if it were bright light." The omission from p. 184 is only that, an omission. It isn't a contradiction.
It's reasonable to conclude that by RAW darkvision is darkvision, and works the same for every creature, unless otherwise noted in the creature's description.
The description on PHB p. 184-185 was corrected in this errata, which says:
Darkvision (p. 185). The definition of darkvision here now matches the definition everywhere else in the game (6th printing).
In summary, the definition of darkvision listed on p. 20 of the PHB is correct, and the omission on p. 185 was corrected in errata.
Best Answer
Dim-light does not impose disadvantage on attacks, only on perception checks that require sight (PHB p. 183), so yes, he can sneak attack from dim light provided he has advantage from some other means.
Regarding lighting, only total darkness (or otherwise being unable to see your target) imposes disadvantage on attacks. So, you could effectively sneak attack from darkness using your goggles, however then you would no longer be hidden, and enemies could make attacks against you (at disadvantage if they can't see) until you hide again. If disadvantage were imposed upon you for some reason (magical darkness, blindness, etc), you would not be able to sneak attack because disadvantage and advantage cancel each other out.