I do believe there are users with 64bit Vista. Microstudio doesn't work with it but Hoster does. You can always try the demo to see if it will work.
Before you load it though I recommend you go through this list and do all the steps.
http://www.mtu.com/upgrades/hoster/v...de-hoster4.htm
These are necessary steps/requirements to get Hoster to work right on Vista.