So the gems of infravision don't seem to work, its still dark without a light spell.

Which characters should I take and let go? Drizzt is over powered and I think he leaves later in the game so should I take Manahath the barbarian and drop drizzt for a while?

Yeah, Vermulean's "can see as well as Drow in the underdark" bit seems a bit oversold. You definitely need light spells down there. At least the gems do work when it comes to protection from radiation.

Which characters you should keep depends on which characters you have. I would definitely let Drizzt go before he forcibly leaves; the sooner, the better. This way other characters can get some more XP.
I always keep Vonar around for his mage spells, as well as Borenorak for his priest spells (even though I generally walk around with a fighter/mage and fighter/cleric; I like to have lots of spells), but if you desperately need a frontline fighter, then Manahath is a good choice as well, IIRC.

Great Thanks Gotcha. Right now I have Vonar, a Cleric, Mage & Manahath. I find spells hard to use in this game due to combat being real time however.

In this game you won't need to use spells that often, in my opinion. Both Ravenloft games (same engine) are a lot harder than Menzoberranzan and spells are more important there.

But it's always nice to be able to nuke things from a distance with a mage. Just run away if something is too tough and start shooting from afar. And rest often to memorize them again.

I mainly use clerics for healing when resting and don't really bother with buffs in this game.
The cleric's Spiritual Hammer is very valuable though (and perhaps even overpowered). You might want to have one ready in the mines. *hint hint*