First of all, usually you do not need to type your IP adress in vlaunch window, because Vavoom can detect your computer IP automatically.
If one computer acts as a server, a player who using it should simply run Vavoom and start a new multiplayer game (Multiplayer / New game).
Another player, who will join, should run Vavoom, go to Multiplayer / Join game / Join game at : then type SERVER PC's IP (not his own) in the text box.
If everything is correct, that, second player should see a list of games run on the server (in your case there will be only 1 game in this list). By selecting it he will join your game.