Lots of things to address here:
-Vampire:
Bit of a problem lorewise with that, I think they've mentioned that vampirism in 2hu in the "kill/drain blood" kind of way just makes a much crappier vampire/ghoul that'll quickly evaporate in sunlight. It won't make a quality one like the current known 2hu vampires/other anime-styled character vampires. Another thing to keep in mind that the two Scarlet sisters are 5 years apart despite looking about the same age which leads me to believe they are natural vampires rather than turned ones.
As for Kurumi, well, we don't know much about her because PC98, but if she's a natural one and not a turned she's probably older than them based solely on default appearance. Correct me if I'm wrong, I'm not 100% on PC98 lore.
In terms of vampire, from what I've seen in lore they're more referred to as "devils" rather than "undead", another used term that pops up is "blood sucking oni", Hence the whole "bean throwing" thing AKA: That command you should disable with the filter just in case since all it does is upset 6 (or more?) characters.
Just found the Vampire lore in Perfect Memento, I don't remember if this is the only place that mentions this: Their main diet consists of the blood of humans. Those whose blood has been sucked by one will neither die nor become a ghost. They will turn into a zombie for a while before evaporating under the sunlight.