Tribes: Vengeance, while being a great game, is very... tempermental. And, just so you know, I had this EXACT same problem, except that nothing worked... It would get on fine and everything, but when I tried to load a single-player game, it would crash (I didn't even bother trying multi-player, but I'm sure it would've done the same thing). I fixed this with a re-install... Sorry, but I think this is the only way to get this game to work right. It's a very interesting piece of software. But hey, look on the bright side, it's only 25 bucks now!
Just wondering, but did your game (when it worked) crash every time you minimized it? Mine does... It's pretty annoying, especially when something pops up and I haven't saved for a while... VERY irritating...
Ah well, I hope you fix it.