Short answer: If your computer is good enough, it runs fine. It will run slower on Vista than XP, though.
Slightly longer answer: On Vista, Orbiter is CPU-limited, so as long as you have a fast CPU, it'll be fine (slower than it would be on XP, but if you have a good CPU you'll still be above 80-100fps all the time)
Really long, technical answer:
http://www.orbiter-forum.com/showthread.php?t=7356
Prelude to a custom answer: what are your computer's specs? RAM, CPU, graphics card?
The important answer: get Doug's VistaBoost from
http://www.dougsorbiterpage.com/index-3.html
See the "really long, technical answer" for details.