My recommendation is that you remove the virtual machine folder from your TM backup list. As has been mentioned, the slightest change to your virtual machine (= any use, even booting it up) would cause it to be flagged for back up, and that means a whole new copy of your virtual machine is being copied. If your VM is 16GB, every single use of your VM would mean another 16GB file on your TM backup drive.
That would fill your drive fast, and rob valuable space from your "real" backup needs.
If you really need the VM backed up as well, I recommend manually backing it up once daily. Before shutting your computer off for the night, manually copy the VM to your backup drive in a folder you create. If you want to be really efficient, partition your backup drive into two parts, and leave a small partition for manual backup content (like your VM or Entourage database).