We played Last Stand today. The Imperial player asked if he could use Sustained Fire on Vader even though Vader can't receive conditions. We all agreed to let Vader use it without suffering the stun condition. While we agreed that was acceptable, something about the wording on the class card left me with room for error. Although I think we played it correctly...

The Rebels won anyways, although it was close. It would have been a slaughter otherwise.

Yes, it is rules as written. Being able to suffer a condition is not a prerequisite for Sustained Fire.