If you have multiple VPNs you might run into the issue that when they connect in random order, their interface IDs change. In that case the normal
ROUTE -P ADD 10.0.0.0 MASK 255.255.0.0 10.0.0.1 IF 42 does not work. The next time the VPN connects it might have a different interface number.
Powershell has a cmdlet available that adds routes on VPN connection and removes them again when the VPN is disconnected: Add-VpnConnectionRoute. It works without having to specify the interface ID.
The basic syntax is like this:
Add-VpnConnectionRoute -ConnectionName "VPN Connection Name" -DestinationPrefix 10.0.0.0/16
After entering this command, the routes will be created/removed automatically on connection/disconnection of the VPN.