Defeating Vamp
First, use the Tranq. Gun to reduce Vamp's psyche to zero. He should pass out. Then quickly equip the stringe/Anti-Nano shot and use CQC to grab Vamp then press Triange. You must be not holding anything but the stringe, otherwise it won't work. Instead of cutting the throat, Snake will 'give' Vamp a shot of the Anti-Nano, making Vamp mortal, which will make him able to die and to stay dead.
Good Hunting!
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