This is slightly bemusing to me because in most of the environments that I've come across for remote users to have VPN access the administrator needs to to have control/management over connecting users to ensure that the network remains secure. That means administrative access, control, etc... of connecting machines and users. This means that the administrator can control the IP address range which means the chances of what you are describing is basically impossible.
That said, your solution does seem workable but very difficult with regards to using different IP ranges.
One option is creating a script that you run on connecting systems to overwrite routing tables to reduce the chances of a possible conflict (am aware that certain VPN solutions are able to do this). Effectively, the organisational network setup will take precedence over the local network setup.
openvpn client override default gateway for vpn sever
This leads to other possibilities. Assuming users don't connect directly to IP addresses you can modify DNS configurations/host file entries so that they technically override the existing local network setup.
Another way is to change your organisational setup to have a less common IP address backbone. Since you have administrative access you should be able to do this quickly and easily (though I've since read another comment which brings IPv6 issue into play).
Obviously, you'll need to change the type of VPN setup you have to give you some of the options that I'm outlining above if you don't already have them.