Just stumbled across some very thorough documentation on backing up Virtual Server with Acronis True Image and Microsoft Data Protection Manager:

Backup and Recovery using Microsoft Virtual Server 2005 R2 Service Pack 1 and Acronis True Image 9.1 Enterprise Edition

Deployment Cookbook: Hosted Backup—Microsoft System Center Data Protection Manager 2007 and Microsoft Virtual Server 2005 R2 Service Pack 1

Deployment Cookbook: Offsite Backup—Microsoft System Center Data Protection Manager 2007 and Microsoft Virtual Server 2005 R2 Service Pack 1

Deployment Cookbook: Onsite Backup—Microsoft System Center Data Protection Manager 2007 and Microsoft Virtual Server 2005 R2 Service Pack 1

Cheers,
Ben