Like I said, it seemed to work for a little while initially...

The setting is on the VPN gateway and beyond your control. They could have changed the setting after you initially had it working.

That being said, the setting is enforced by the VPN client software on your machine. It might not be intelligent enough to deal with two NICs, it depends on how it locks down the connection. Good client software would be able to do it.

First thing you should do is ask the VPN admin whether this feature is enabled or not. Pointless banging your head against a brick wall if it is.