Well, by design pfsense dpinger and related routing table updates of bsd won't let you use the same ip address. A routing table for specific IP is just that: it allows an exit on a specific interface. If you connect to your vpn provider through different servers , a new gateway is created for each connection . Also, If your provider happen to give you the same IP for 2 or more connections , it might be game over conflict for connectivity tests and maybe gateway status. The solution to your problem is not to monitor the vpn gateway ip which is the same on every server , except the first connection, but choose a well known ip , e.g 1.1.1.1 or 8.8.8.8 as monitor IP for each vpn gateway. If you need to compare vpn connections , it will not be a stable basis for comparisson , as the external ip will have longer ping times by a 25% margin approx. I understand your concern from a paranoid security point of view, as pinging a vpn gateway does not leave any traceable exposure on vpn exits for your pings..where advanced adversaries might interfere with..
The limit of monitoring with a single IP the connection status tries to tackle a new advanced plugin which is under development for the time being..Since then, try, to diferentiate your monitor IPs for each gateway manually..