Everyone, READ THIS!
the easiest way to kill Vamp is to take out his life once as fast as you can.
after that he will get up. as he gets up and does some stretching stuff, that will be the time for you to do some CQC on him because he won't really do anything to you then.
finally.Vamp dies. sorry if this is a spoiler but Raiden loses his arms but after you use metal gear rex to beat metal gear ray(controlled by Liquid but then he gets like mechanical ones after and he finds out that he has a son named Little John.
(note:fighting metal gear ray could be the easiest thing in the world because all you have to do is shoot the face or after it blocks, you can hit it anywhere(easiest boss so far)