Are you using Wireless Zero Config? If not, you might want to try that. Also make sure you are running SP1 on the Vista machines.
What is your session timeout for the WLAN? If it's set for the default, which I believe is 30 minutes, then you might want to change that and see what happens. Yeah, I know, it's 30 minutes, not 15 minutes that you're seeing, but I had a similar problem to yours that went away when I changed the session timeout.
You might also want to check your DHCP lease duration (and no, an expiring DHCP lease should not cause a dropped connection, but stranger things have happened).
Do you see any messages on the controller about why the clients dropped (or other messages that were logged WHEN the clients dropped)?
If it matters, I run Vista SP1 with 188.8.131.52 and 184.108.40.206 all day using an Intel 4965 and the latest drivers. No disconnects.