I haven't tried VPN and edit. I know my boss does without any issues. It could be a vpn setting though.
Have you tried using a remote machine at the office and connecting into that? At the very least it could be a stop gap until you figure out the main cause of the problem.
I would say this is probably not FMS but network/vpn latency
We have used Filemaker 14 in combination over a VPN connection for 5 months or so now, no problem.
This has probably to do with the size of your application.
In our application it takes about 20 seconds to open the database when we start editing scripts.
Just wait ...