You basically just have to prioritize doing whatever you can to survive. If that means you can't attack much, so be it. Let your team peel for you and kite as you can. Against a vi and riven, you may well have to bank on one teamfight to win the game, and avoid fighting as much as possible, because your summoners are going to be the only way to make it out of a fight alive. Buying a ghostblade might not be a bad idea for the MS active. If they keep focusing you, then as long as you can run, you can potentially act as bait and let your azir do some heavy damage. Rejoin the fight once it's safe and hopefully clean up.
Of course, that applies to the micro level of the teamfight. For the macro level, there's another factor that plays into winning in this situation, and that's the split push. Is trynd able to 1v1? If there's someone he can't 1v1, is it only one person, and can he get away from them without dying? If so, send him split pushing while the rest of you play defensively, and try to force a 4v4 while they send one of their bruisers to stop trynd. If it's only vi or riven gunning for you, it should be much easier for you to survive.