I don't think that 6 and 7 can co-exist. I would:
back up the VM
uninstall PD 6
install PD 7, this will update your VM (in particular the VM specific tools and drivers)
play with it and see how you like it
If you don't, uninstall the PD 7 tools from the VM (using Windows' uninstall feature), shut the VM down, and then uninstall PD 7 (but keep your VM files!). Re-install PD 6, open the VM, and install the PD6 tools.
If all fails you'll still have the PD 6 backup.