This is the fastest Vamp battle I've ever done. Start by shooting him in the head, then after he dives into the water, shoot 2 stinger missiles to lower his stamina a bit. As usual, go into hanging mode at the edge of the pool. This next part requires very lucky RNG, as you want him to jump to the right side of the pool so that you can engage him in battle much faster. If you time it right, you can get out of the pool, with him slashing you near the very last i frame you have while exiting pool. At this point you can hit him up to 6 times before you need to move to avoid him slashing you(which is instant death btw). Then you can hit him up to 9x before moving to avoid him again during each cycle. If you time the hits right, you will not need any more than 6 full cycles to defeat him. Be wary of his stamina bar. If the last hit is the final kick of raiden's full melee combo, it will end much faster.