If you're referring to the fight with Nazuna

No that's Vol11 IIRC. I'm talking about the fight where Kou literally destroyed a building. He wasn't in any particular emotional state. He calmly talked to his opponent. Then he became super powerful (reattached his arm by using his blood like a rope) exactly because of the pain element.

The whole "emotions are the trigger" was a wild assumption by Detective, who quickly retracted and then formulated a better one in Vol12 based exactly on the pain and loss of blood requirements.

The fact that in the latest chapters he could turn into vampire mode in order to escape the hotel, while he was just simply worried about a classmate, kinda confirms it.

I enjoy the shift and how things have adjusted, we all knew that if he would eventually turn , that he would have the physical strength of a vampire which is like old school dnd levels of power.

My point is that the manga was never about punching and kicking enemies. Sure, vampires had superhuman powers and strengths, but those weren't used for fights (if not for very brief moments). The story was more about different elements. Now it's shifting in a "good guys vs bad guys with a sad story" and I don't know how much I'll like it. Vol12 was quite disappointing to me.

So long they keep the fights toned down, and the mystery going, I think I'll stay.

And, does it have to be just one weakness?

I don't know. I don't think even the author knows. First he makes Detective talk to herself saying "Vampires have many weak points" and then he makes Detective say "Vampire have just one weak point". As I said, there are small plot holes here and there.