VirtualMac2009 wrote: The problem is on one of them it has been left idle since it arose.
Not sure what you mean by that however that doesn't clearly answer the question I asked! 
Do you know if backups made with SuperDuper! could be safe?
If you are talking about a whole disk image of the Host made while the Virtual Machine was shutdown, preferably not suspended and VMware Fusion closed then the copy of the Virtual Machine should be okay to recover from. If you're talking about an incremental updated backup then I wouldn't count on the Virtual Machine to be okay.