Not sure if you can force other NICs metric setup to any value you desired.
Basically, a VPN setup first need to have client to reach the VPN Server via public network or any common unsecure cloud.
When a VPN connectivity between client and VPN Server (PIX/ASA/VPN3K?Router) is established, logically they are sitting/located in the same network/LAN segment where the VPN Server resides.
The metric value of 1 is set automatically as the client initially is only 1 hop away from the VPN Server.
HTH
AK