No longer glitched, the game awards MVP medals when you place top of the match leaderboard.
It should unlock right as the game announcer says "game over".
Play in public matchmaking, do the best on your team and win. Simple enough right?
Can not be in a private match.
A tie with another player on your team will give it to all players with the shared high score.
It's possible to boost this by going in with a full party of the maximum amount of players.
Getting disconnected mid-match may also award you this. I wouldn't suggest trying to cheat by shutting off your internet, that could be grounds for a ban from Halo matchmaking.
You score points differently based upon what mode you're playing.
Team deathmatch is kills, Capture the Flag (CTF) for flag captures, various other capture/hold/territory modes for capturing/holding.