I was playing the fourth Snake Tales mission, beat Vamp only to find out I got the 'bad ending'.

Well, hell. I don't think I did anything wrong, I just shot him senseless. Exactly what is the good ending and how do you achieve it?

Beat them all once then go back and you'll have the M9

Ah, I see. So tranq'ing Vamp gets you the good ending after that? I see.