If Relic Vial leaves the battlefield at the same time as each Cleric you control, the triggered ability triggers for any creatures you control that die at the same time (possibly including those Clerics).

Once Relic Vial’s ability has triggered, that ability resolves even if you no longer control a Cleric.

In a Two-Headed Giant game, Relic Vial’s ability causes the opposing team to lose 2 life and you to gain 1 life.

If you sacrifice the only Cleric you control to activate Relic Vial’s first ability, the triggered ability it gives itself triggers.