To enable paralel backup for VM, you need to have Hardware VSS Provider. I have discuss about the problem that you will face if you did not have enable paralel backup. For more detail, please refer to http://www.ms4u.info/2010/08/event-id-1584-backup-application.html
If you're interested to know which Hardware VSS Provider has verified and tested, please refer to the link here:-http://blogs.technet.com/b/dpm/archive/2010/07/08/tested-hardware-vss-provider-table.aspx
