You can basically duplicate the "internal" profile and then set the: ext-rtp-ip, ext-sip-ip, rtp-ip, sip-ip to the ip of the VPN server (ex. if you use 10.8.0.0/24 network then the ip would be 10.8.0.1). Restart freeswitch.
The sip profiles are connected to the IP addresses so for each...