Running virtualhosts under different users

I realize this is more of an apache question, not a php question, but I thought you guys of all people, would be able to help me... I have an apache server setup, hosting a number of sites. I need one site to be ran under a different user so it can execute shell commands via system() under that user. Does anybody know how I can do this? I installed the suexec module for apache, and either it doesn't work the way I need it to, or I just misconfigured it. I'm baffled, any information would be greatly appreciated. Thanks -Adam