Nobody else has mentioned a performance drop in Vista, so it may only happen with certain combinations of hardware / drivers / software.
Did you try setting BD to Win98/ME compatibility mode?
A few people had sound problems with Divine Divinity on multi-core systems which in a couple cases could be fixed by setting the game to run on a single core. It can't hurt to try this for BD.
see;
Sound errors with static - maybe fix found! 
