Well, if you can't join any games, then it could be because the Vista firewall is blocking outgoing communication from the game. Both Lithtech.exe and TRONSrv.exe need full permission or need to be trusted applications, so they can access the internet. You'd go the the Control Panel, then Security Center, click on the Windows Firewall icon on the left side, and then click "Change Settings". From there you'd click the "Exceptions" tab and add the two programs if they're not there, or change their properties if they are there.
It could also be because you've got too many map packs, or you've got a mod running.
For hosting games, there's already a long thread about it that you can check out. The same advice for XP applies to Vista. The built-in firewall didn't change much between XP and Vista.
http://www.tron-sector.com/forums/default.aspx?do=top&id=357556
