For anybody who was having this issue, I was as well but I figured out a solution that worked for me.
Ultimately its a permissions issue with Vista and to fix it I went into the BitDefender directory and set both bdagent.exe and vsserv.exe to run as admin in the file properties. after setting them I rebooted and presto, anti-virus worked. You may not have to set bdagent.exe to run as admin but for me it didn't work until i set vsserv.exe to.
So good luck to all. 