
So I guess there is something about Sooin's blood that sort of weakens or negates a vampire's ability and so maybe by constantly drinking sooin's blood chi-hwan was becoming a less of a vampire/losing his abilities. The other guy does mention that it was a poison which could mean it was slowly killing chi-hwan (making him mortal/human) and the one last thing chi-hwan did before he was going to die was turn sooin into a vampire and once chi-hwan was close to his end/mortality sooin made him his link...?
"You come back naughtier everytime I send you far away" has got to be the best line