And what if you don't get the indicator that you need to press 'B'? If you're just standing still and Vader tries it, then you get the indicator to press 'B' and about two seconds to beat the attack. If you are doing a charged Force repulse and he does the attack, then you beat the attack there... he can't beat you when you've already negated his attack!
I know that there are certain discrepancies between player and NPC, but when it comes to Darth Vader... you have his abilities in the beginning, yet they change the second time around. If you could use choke on the Rogue Jedi at the start, then I would assume Vader can do that to Starkiller. If the Rogue Jedi can ALWAYS beat choke, then so should Starkiller!
(I know this is just a cheap way to inflate Vader's pathetic abilities for the second-last boss fight. I don't really expect anything to work properly.)