I use VNC to access remote users via VPN and it does work OK.
Make sure that the VPN allows the network you connect from, to whatever IP subnet their client has. I would first check you can PING them, which means the route is there, and then try VNC. Also check their firewall allows this, probably first worth trying with the two machines on the same LAN, if that is possible, and then try it remotely.
I’ve used both teamviewer (non-hosted) as well as logmein free version (hosted). Either product would suffice for any users.