According to the Halo 2 Vista FAQ -
Q: Will we have to pay to play Halo 2 Vista?
A: For a free Silver membership, no, you won't. With a Silver membership, you will be allowed a single gamertag, a common gamerscore and and gamer profile, single player achievements, private chat via text and voice, a common friends list and online presence, and a browsing list. With a Gold membership (which you do have to pay for), you have everything the Silver membership offers, matchmaking, and multiplayer achievements.
So it sounds like you need a gold membership to play online, but I'm not sure because I never tried to play with a silver account myself so I may be wrong.