If you're using Windows, you might notice that when your VPN disconnects for some reason, traffic will continue to flow and your real IP can be exposed (at least, this is what use to happen to me under WinXP). To avoid this, you could use a linux virtual machine and start your VPN through the command line. If the VPN fails, the connection will freeze.
You can also tunnel your VPN through TOR (a tunnel within a tunnel). You'll need to alter your VPN from UDP transmit to TCP in order to get it through TOR. Routing will be the tricky part. The VPN will try to encapsulate all traffic, including what has to go through the onion router. It can be done, but it's really slow.