[Serusers] ser and firewall on same server

Dual homed server, operating as a vpn server, has one public and one private ip. Remote client pc's ( scattered around the internet ) connect to server's private ip (via openvpn) tunnel, server NATs traffic originating from private/vpn interface through server's public ip off to the internet at large; all this works fine: pc's route all traffic through vpn. Now, if this server is also running an instance of ser, via which the client pc's register - therefore, the NAT'ing takes place on the server _itself_, rather than on some intermediary server/router _between_ the pc's and ser server - is it still necessary to run mediaproxy or nathelper? I'm still cutting my teeth on voip in general, and ser in particular - however our environment seems quite different to what I have seen and read in all the docs and material regarding sip/ser and the NAT issues involved -- all of these seem to assume that the NAT'ing takes place on external/3rd-party servers/routers rather than on the ser server itself. As such, I'm having all kinds of fun trying to figure out what the heck I'm doing - whether I'm on the right track, and am merely suffering from some small misconfig somewhere that is preventing all this to work, or whether this scenerio is so outlandish that I'm unlikely to ever succeed period. So what I'm hoping to get, is some indication from you experts on whether this environment I've tried to conceptualize above is possible or not; and any tips or suggestions you may have. I'll be happy to provide further clarification on anything that I haven't described clearly. Many thanks! Cheers, Corey