EDIT: Was typing my reply and missed your update.
The best course of action is to not use a VPN with Plex.
If you need the VPN for other applications, find one that supports Split Tunneling. That should allow Plex to bypass the VPN.
When researching, make sure the VPN supports split tunneling when installed on a NAS. Some VPNs support split tunneling on Windows, but not on NAS or other systems running Linux.
The Basics of Remote Access Troubleshooting
Plex Documentation → Troubleshooting Remote Access
The VPN is absolutely part of the problem. Disable it (or better, uninstall), then restart Plex Media Server. Plex remote access will not work with the VPN running.
Also, the servers must have different public ports in the router port forwarding statements. They cannot both use 32400.
You can use 32400 for one of the NAS. Pick a different port (ex: 33400) for the second NAS.
On both NAS, you must manually specify the public port and it must match what is in the respective port forwarding statement in the router.
Example using a different public port.
Asus router port forward statement
Plex remote access settings
