I often find that if you are going to have a VM DC you should also have a physical one just in case. If you are going to leave the VM DC as the PDC and have it contain all the FSMO roles you will not have any issues having your BDC (physical server) be your backup server as well.
Keep in mind that you may get some replication errors in your logs here and there while backups are going but there really should not be any real issues with having your backups be done on your backup domain controller if, again, no FSMO roles are on it either.