Great question — answering as the UphillTech team:
Most rebels would recognize Darth Vader in the same way that many WWII soldiers would have recognized Hitler, even though they never saw him in the flesh. Photos and verbal descriptions would be enough. But Han had seen Vader fighting Obi Wan on the Death Star, and then he joined the rebellion for several years. It is a near certainty that during this time someone in the rebellion would have briefed Han up on the giant guy in all black armor that he saw, assuming of course that Han didn't already know who Vader was since presumably someone that high up in the Empire would be relatively famous. As to why Han's reaction would be to draw and shoot as fast as he did: instinct. Despite what...
Full answer by ruffdove: https://movies.stackexchange.com/a/111531 (CC BY-SA 4.0).
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