Like Kiku was out in the daylight when she led Kou to his mom
She took out a parasol when the sun came out (chapter 129, p. 17-18).
and I mean, sure, maybe we can start playing "there's a certain AMOUNT of sunlight that is required to be harmful" but then that would just kind of suck.
We don't have a choice-- Kiku talks about how Mahiru should stay in the shade as much as possible and how it won't have an immediate effect but he should still keep in the shade (chapter 163, p. 13).
In truth, no vampire has outright said "the sun will kill us", but they never act like the sun will merely give them nasty sunburn. Haru and Kiku agree that "it really sucks" (chapter 146, p. 7) in a conversation involving examining vampire legends... to find a potential way to go back to being human.
Kotoyama's so evasive about this that I can only imagine there's some yet-to-be-seen narrative purpose for it. Or maybe it's a "no shit" kind of deal, given that vampires also can't see their reflections in mirrors in this narrative (a "conventional" vampire trait).
Edit: It's also worth noting Miss Detective attacked Akkun at night with the intent to kill him
She didn't-- the first time she attacked Akkun (chapter 55, p. 5-12), she wanted to interrogate him after incapacitating him. She also attacked with Hatsuka right there, knowing full well that he was also a vampire. The second time she fought with him (chapter 86, p. 7-10) was actually her getting him off her tail, since he wanted to wreck her for (non-fatally) shooting Seri in front of him.
In the first place, Kyouko's vampire hunting was significantly suicidally motivated-- at the least, she was reckless, but she actually tried to blow her brains out after her plan was foiled and she lost interest in her goal. But she definitely intended to be a martyr whose death would reveal the existence of vampires and accordingly keep humans off the streets at night (chapter 90, p. 12-16).
wasnt the fuck boy that asked nazuna for date was in the sunlight the first time he was introduced? trying to pick up Kou's classmate?
Haruka only showed up when it started getting dark (chapter 139, p. 12-13).
This kinda feels like some important information
Or Hatsuka could be
talking out of his ass trying to make sense of what was reported to him, given that before Hokkaido, Susuki was (between the instinctual aversion of a vampire to sucking the blood of someone they've fallen in love with and what's likely an vampire urban legend constructed around that) of the understanding that the
candidate would die, with no reference to the vampire becoming human (chapter 122, p. 6).
Kou seems to be losing interest in the whole vampire world
Even if I looked at him as cynically as possible, I don't think a fourteen year old boy would lose interest in being able to be intangible, fly, and have super strength.