... actually, from what post in tumblr I saw, I learned that vampires couldn’t be killed by the sunlight, they just made them as vulnerable as humans, but killing them? Sorry, but that’s impossible....
(The real question is, who’s bedroom it is? Mostly because it wouldn’t be Flan's, cause her bedroom are located in the basement, and I doubt that sunlight could come in there, nonetheless having windows... and while Koishi’s also a possibility, i think it would be weird to have daylight in the underground, where everything is dark...)