Some routers have the ability to use a vpn built in, so all network traffic goes through that vpn, you should check yours. Also, what you could do is connect to the vpn via your pc, connect your ps3 directly to your pc, manually assign the ip addys, and bridge the connections of the vpn and your ps3 to pc connection.
Although I suspect you want vpns for "lan" (internet) play, in which case you should just use xlink-kai, or xbslink, which is a lot easier to set up, and probably a better option if you just want a private server with a mate.
Search button is your friend.
STOP! Before you ask questions... READ the first post! RTFFP = Read The F*cking First Post
Yes, A 3.55 "jb" (cfw) is available, no >3.55 "jb" or cfw yet.
Yes, A 3.55 downgrade is available. But not to lower than stock firmware! (What it came with)
No >3.55 downgrade is out there yet.