Mosquitoes are persistent bitches. They will come back. If they dont come back, it is usually the new ones that come seeking fresh blood for nourishing their new eggs.
Fun fact: They have sort of detector for nearby living bags of blood they want to gain the red juicy nectar from. They detect carbon dioxide which we exhale out.
I'm not proficient enough in japanese to understand the subtle details but someone who's totally not mizore is basically asking "Drinking tomato juice instead of blood, are you crazy? The OP should investigate vampires more." (not sure what 描けよ actually implies here though)
Rin is basically answering back "What do human (nature) know about vampires? Don't listen to this guy who's just acting like he knows shit" (ニワカ here is a mystery though. Only thing I'm getting on it is 俄 which just means "sudden." Might me some kind of message board slang?)
I think "ニワカ" here is Internet slang for "にわかファン" meaning "temporary fan," i.e. a trend follower with a shallow understanding of the subject.
I highly recommend NicoNicoPedia as a resource for translators. It has saved my ass many when it comes to net slang.
maximuffin2's post of that OPM gif just made me realize something.
Can that mosquito even penetrate Saitama's skin? It'd still be annoying landing on you and shit but eventually that thing's bound to be like "Man I can't do anything with this is someone else around?" 'cause I don't think getting hit as hard as he has and not even being phased by it means a skeeter is drawing any blood.
Moral of this story is: if you are a yokai allergic to the sun, you need a human child to look after you, because it is likely you're also an adorable idiot