Now that higher priorities numbers really mean more important,
it seems that the VPN configuration should be rather important.

Bump the number, also in relation to NMDevice's L3ConfigDataType.

It might not matter too much, because usually the VPN tunnel device does
not have NDevice to add other l3cds and those from VPN might be alone.
Except, maybe with routing VPN (libreswan) that is different. Dunno.