Sorry for the resurrection, especially from such a beloved OP, but I have been having this issue as well. On my PS3, on Warrior, it was extremely hard to shake off Vader with button pushing. On Master, it is, I believe, impossible. I was a NES Track and Field champion, so I feel confident in my button mashing skills.
I want to know if any other PS3er's are able to shake off the choke on this difficulty, or if it is simply designed to be that way on Master/Lord difficulties. Reflecting Shem's post, I have no problem with it being a tough match... and I also have no problem with it being impossible to shake, since there is a way to avoid it in the first place. Just wanting verification, I guess, that I am not slipping and becoming feeble :|