Responses
I was confused too but upon rereading I believe that Soo-in's blood didn't kill vampires, but simply took away their powers and turned them back into humans. So vampire Soo-in was able to make newly human Chi-hwan his link to save his life. Happy end!
I am really confused about the whole linking and unlinking stuff but I am pleased with the results(づ￣ ³￣)づ