NWN2 and Mask of the Betrayer run fine on Vista. Both 32 and 64 bit.
The problem is Atari/Obsidian have a horribly outdated game engine and lousy Quality Assurance.
To the OP: If you have put anything in the Override folder to customize the game - Take it out. You can rename the folder and make a new Override. That way you don't lose your mods.