Hi,
The question is known . Check pfsense API.
Not something for tomorrow, it's a huge job, and needs an entire GUI internal rewrite (like the GUI will be using also the API to handle ALL settings).
HP code and passes in the arguments? A huge hassle I guess.
A local scripts that read the concerned VPN section in the config file, changes, sets the Disable flag for one VPN server, and resets (removes) the same flag for another server. The write back your changes. Then a "reload_filters".
Maybe you should stop the VPN server first - do what I said above, and start VPN.