Sometimes Vista doesn't use all your cores. Same with w7. It happens with stock computer too, the manufacturers don't turn on all the cores..
Go to start > Run > type in "msconfig" > go to "boot" tab > select "advanced options > turn on your cores by selecting however many cores you have.
I'm pretty sure that only controls how many cores your computer uses when booting up...
I think vista does do a good job at handling multicore, most things I use have all the cores working although one can be 100% and the rest lower when using nero all my cores are maxed out when video converting. I was thinking about going 64bit still worried some programs may not work..