Check out this article by Chris Wolf on MCPMag.com. This article contains a great step-by-step procedure on backing up live Virtual Server virtual machine files even if your back up software does not support Volume Shadow Copy Service (VSS).
As I have learned firsthand, backing up virtual machines can be a problem. If the back up software is not VSS aware, it will cause your virtual machines to crash suddenly. The alternative is to stop the virtual machine and let the backup software copy the closed files. This is also not desirable if the services on your virtual machine (i.e. web sites) are not meant to be down for any significant period of time.