As is the case with many games, in W:A you cannot join a LAN game on a different vLAN then the one your computer is on. some games like blizzard games have a 3rd party solution called battleLAN that lets you input the hosts ip address allowing you to join their game. Is there some option in W:A to join a specific ip address or a external applicaton that fulfils the same function?

Yes; in the more recent betas, W:A features direct-ip play, so if you know the other people's IP addresses there should be no reason to set up a vLAN. You can also set up passworded games in WormNET, if that's giving you trouble.