elo/mmr matchmaking

What is going on with matchmaking? Just played a game with my buddy in team builder. I'm silver 5 and he silver 4 the rest of our team was unranked and on the other team was a plat 3 Vayne player. Like what the hell riot? How is that fair and balanced? She carried her entire team we lost the game after being ahead all game till Vayne hit late game; and in the hands of a platinum player we got destroyed. So much for fair and balanced matchmaking.