My players recently dealt with battle-in-the-fog. One fight, with 5 PCs against 13 lower-level soldiers, took about 6 hours of game time. Most of that was because everyone was limited to 10' visibility, and the party split themselves up. So I was tracking everything behind the screen, and was the only one who knew where anyone else was. It did take more time, but after, they said the suspense was great, and it was one of the best battles in a long time. I should add, it was a PC who cast the fog spell as a defense. She was flying, too, so I actually had to track her movement (and that of two enemy NPCs) in 3-D.
One thing RM doesn't specify (I use RM2) is the time it takes a vampire to change form. Nor, I guess, does it say how often they can change. I would make some decisions on that in advance, too, or it could be overpowering.
You mention the sorcerer transporting the barbarian into the lake. If you're thinking Long Door/Leaving/Teleport, bear in mind that RM classifies those as Utility spells and they aren't supposed to be castable on an unwilling target. (Otherwise, why transport him in a lake, transport him up and drop him.)
If the sorcerer is the source of the mist/fog in the cavern, you might consider the Fogvision spell (idk the list right off hand) for them. That would be a powerful pairing.