Birds are the Light Side and the Rebel Alliance, pigs are the Dark Side and the Empire. As such, Darth Vader is a pig-equivalent. Behold the trailer:
At 31 seconds in, the gameplay footage clearly demonstrates Darth Vader's pig-itude.
Additionally, at 23 seconds in, this adorable "unboxing" video of Hasbro's Angry Birds Star Wars toys shows the packaging labels Darth Vader as "Darth Vader Pig":
Moreover, the game only includes levels that feature:
- The first Death Star encounter
- Hoth (eventually via a free update)
- Dagobah (via in-app purchase)
Meaning it stops around halfway though the plot to The Empire Strikes Back. If there is an eventual redemption for the Darth Vader pig or a twist ending regarding his true nature, it's not in this game, or at the very least, not in this game yet.