Peachtree (now Sage 50) uses the Pervasive database engine...sorta a twist on SQL.
So, there's data "up in the air"...when Peachtree is running. Even when it's not.
With transactional database engine based programs, it's best to have the program itself spit out copy...and then your back programs sweeps through and grabs that backup file. I can't remember what Peachtree called it, .PTB file? Anyways, I'd make sure that is in place.
Heck, VirtualBox supports USB device passthrough, so you could have it save to removeable USB drive. I'd trust that a lot more than doing a VSS backup of a guest that never shuts down, and that has a database engine constantly up in the air.