Why do you still boot Vista?
I don't know if it's possible. I think there are registry settings that Steam requires, and you'd have to install it in one OS, leaving the other without the keys.
You may be able to install Steam in both, then install the games you want in both, then delete the data in one and link it to the other directory using a symlink. But Vista and Win7 have permissions issues and you will have to reclaim ownership of the directory each time you use it (I believe).