Episode IV begins with the Star Destroyer Devastator (Vader's flagship at the time) pursuing the Rebel blockade runner Tantive IV. The blockade runner is soon captured and boarded by Imperial stormtroopers and Vader himself.
Did the Rebels know or suspect Vader was on the Star Destroyer before he boarded the blockade runner? In other words, did they have any idea ahead of time that Vader would step through the breached door?
I'm not suggesting that the Rebels should have surrendered rather than stalled for time by resisting the Imperial boarding attempt. I'm just curious if they knew or suspected that they would have to resist Vader in addition to stormtroopers as the Imperials attempted to board the blockade runner.