I play Agression (Action: Each player must sacrifice a unit, if able.)

Darth Vader has (Reaction: after you play a Sith event card, deal 1 damage to a target unit)

Does the reaction go off first before we each have to do the action?

It makes a difference cause I may kill a small unit with Vader prior to an opponent sacrificing a unit.

sorry if dumb question and thanks for answers.

I believe the played event card would need to be fully resolved to satisfy the triggering condition of playing an event card.

From the Official FAQ Page 7, Column Right, "REACTION ABILITY TIMING", 3rd Paragraph, first line:

"A reaction ability is always used immediately after its

Thank you. Makes sense.