Tor is TCP only, no UDP.
So to get a VPN setup to work, you want to go into the config file and change 'udp' to 'tcp' (it is next to the domain name of the server you connect to, and right next to the port number 443 which you leave intact as-is).
Once that is done, you comment of the lines in config which refer to explicit-notify exit and reply-window since these are UDP-specific settings which are not applicable to TCP.
After that, you can leave the rest intact. It should work. It does for me.
I sometimes use VPN over Tor, but like parityboy asked, the big question here is "why"... Basically, it boils down to two possible answers:
a) you don't trust your VPN or you are paranoid about someone monitoring connections to the VPN and tracing those back to your home/workplace. In that case, with VPN over Tor, the VPN (and anyone watching) will only see that the connections to the VPN come from a Tor exit node.
b) you connect to sites where you need them to see a single consistent exit IP and one which is not a Tor-identified exit node. A VPN (and that includes a VPN over Tor, of course) provides that. Tor, which changes exit nodes every session and every ten minutes, does not.