C3PO is on the LS table, Vader is on the DS table. During DS turn force choke is played. In what order is Vård and abbilities resolved?

1. C3PO is canceling the event effect and nullifying Vaders ability.

2. C3PO is canceling the effect but gets killed by Vaders ability.

3. Event card is played, not resolved, triggers Vaders ability and kills C3PO and then the event card is resolved.

Vader card says when event card is played and c3po says cacells the effect not the card.

You use C3PO to cancel the text of Force Choke (by discarding C3PO) but a Sith event was still played. Vader still triggers another damage to put on another unit.

This is outlined clearly on page 3 of the current FAQ, if you want to be able to cite this at the table. I copy/pasted the text below for reference. Also, as an aside, it's really worth printing out a copy of the current FAQ/Tournament rules and keeping them in a little folder or binder that is readily accessible if you're playing at a game night.

The rules as written in the rule book may not address that fully, and it becomes different people offering their interpretation on that issue, which is fine when something comes up that isn't covered, but why go through that process when you can bust out the FAQ document and solve it in 2 seconds.

(3.1) Cancellation of Effects

Effects that are canceled are still considered to have been initiated or played. Only the effects are canceled. Any costs have still been paid, and are not returned or refunded.

Also relevant to the above scenario is the fact that Vader's ability is a reaction and C-3PO's is an interrupt. That means C-3PO's ability will happen before Force Choke deals damage because interrupts happen before the trigger fully resolves. But as mentioned above, the evet card is still considered to have been played. Since the effect is finished when C-3PO cancelled it, Vader's reaction now kicks in since reactions occur after the trigger is resolved.