So much paranoia. Are you a drug lord or something?
Anyway, you'd have to ask the administrator(s) of this public server directly. I don't see why you would even want to play games through VPN, since it's likely to give you a slower connection.
As for making your own server: Check the Terraria game installation folder. There's an .exe file in there called TerrariaServer.exe. There's also a file called serverconfig.txt, which you can use to set up your server. It's not needed to set it up, but it might be handy if you want to automate things.
If you're playing with "friends", what's the concern?
Any VPN will constrict your bandwidth somewhat, as Gotcha noted, and thus impact MP performance. The Radmin VPN seems adequate (sample review), but it's 'free' and based in Russia, so don't count on it to protect your privacy. I've never heard of it before so I can't add any pros/cons for its use.
If you're connecting to a server as a mobile client, then only the person hosting the server will have access to your IP address. If you are hosting a server (either from a PC or from your mobile device), you will have to share a static public IP address so that people can find you (and a VPN will often rotate through a pool of public addresses, making it hard to host with it). If either of those situations make you uncomfortable as far as sharing that information, then online MP probably isn't for you right now.
eta: I guess I assumed this was about mobile playing, but it applies to any MP configuration. Also, since this doesn't really belong in this forum section (Dedicated Servers), I'm going to move it.