You needed to win ten Ranked matches straight. You could actually quit out of matches by pressing
and closing the game to keep your streak if you were losing/about to lose. You could also boost with another play/yourself with a second Vita if you both tried to search at the same time you had the chance of being matched together. The boost method wasn't 100% accurate but it helped. If other players ragequitted on you that win did not count towards the streak but you kept your current streak alive.
However due to the GameSpy shutdown the Vita version can no longer play online. All other versions of the game are still up, only the Vita's servers were taken down and it's unlikely they will ever come back.
The PSN store page makes note of the servers and that some trophies may be unobtainable.