I’ll be honest, this is something I’ve never attempted. I just backup VM’s at the guest level mainly because you need to do this very often to get the complete results that you need. Things like non-VSS aware applications and granular recovery require in-VM backup/recovery. There’s a few other architectural reasons I don’t do host level backup but there’s one or two other people out there who have done this and are more qualified to blog about it.
However, MS Virtualisation Program Manager Ben Armstrong has done it at home and blogged about the process he had to go through to resolve different issues. It’s a good read, even if you aren’t going to use WSB for your back process.