Activate Warren Soultrader by paying 1 life and sacrificing the persist creature.
When the persist creature dies, its persist ability triggers, returning the persist creature from your graveyard to the battlefield with a -1/-1 counter on it.
When the persist creature enters, Leyline of Vitality triggers, causing you to gain 1 life.
Resolve the Warren Soultrader ability, creating a Treasure token.
When the Treasure enters, Mm'menon triggers, putting a +1/+1 counter on the persist creature.
Repeat.
This elegant loop transforms the life buffer from Leyline of Vitality into an engine of endless resources. By using Warren Soultrader to sacrifice a creature and generate a Treasure, you fund the cycle while offsetting the life cost through Leyline of Vitality's entry trigger. Meanwhile, Mm'menon, Uthros Exile watches those very Treasures arrive and continuously places +1/+1 counters on your creature, neutralizing the -1/-1 counters that would otherwise end its persistence.