You're screwed. MS disabled a lot of the stuff that would make Vista backwards compatible, for who knows what reason (other than they hate people using resources not made by MS).
I don't expect they will make it backwards compatible in the near future either. Until someone hacks Vista (yep, you're going to have to rely on the Pirates that MS is supposedly trying to discourage) and makes a program that emulates the stuff cut from Vista...you might be out of luck.
Or you could try hacking Vista yourself. I don't know what the programming of Rainbow 6 runs on so I'm not certain whether it's just the drivers of your card, or if it's actually MS's "great" gaming advancements that are causing your problems.
See if your card has updated drivers for Vista. That MIGHT work...or then again...maybe not.