Doesn't vampirisation need exchange of blood, or something?
What is the full process then? Flan should know that at least.
Flan wouldn't, actually - she hasn't met enough candidates for the process (that didn't end up splattered on account of they're invading her home ...) that she would have experimented, and she hasn't thought to look it up in the library since meeting Reimu and Marisa. Remilia knows a few ways, but she isn't particularly interested in increasing the number of vampires. For the record, though ... Hmm. Barring other magic (Wishes?):
* Direct and massive infusion of vampire blood; reducing the victim's blood helps, but isn't necessary. This creates a very weak vampire, because it's essentially trying to get the vampire blood to 'latch on' and create an environment capable of sustaining it, which ends up making the body a vampire's. It takes several days for the change to be complete, during which time the victim will be very sick/comatose, depending on the amount of human blood still in the system. It's not a guaranteed method, but it's one Flan could do immediately.
* Similar to the above, but using magic to amplify the effect/weaken the body so it's safer and requires a smaller transfusion. To transform near-humans or something like a Magical Girl, you would need at least this. Flan could do this immediately, but she isn't a talented magic-user, so it's probably a worse idea than the first method.
* A sufficiently complex/powerful ritual could be done to forcibly change something into a vampire. The majority of existing ones require more magical skill than Flan has, and while she has the power to use the few that are simple enough for her to do, they're almost impossible to find. This is effectively sealed off, since she would have to find a ritual first, and those won't be lying around in other universes.
But Flan doesn't know any of this. She has read
Dracula (the wonders of an education!) and a lot of the myths about vampires, but she knows that at least some of those are false (religious imagery doesn't bother her or Remilia, they both have reflections, etc), so she doesn't know if any of the 'traditional' methods work.
(Also, I didn't expect SV to try to make vampires in this. I'm not sure why the idea didn't occur to me.)
Wonder if we can sense Witches or not?
Witches and Familiars are visible to her, and she can see easily-enterable spots on their Barriers, but she can't detect/sense them like Mami does with her Soul Gem. Something radiating magic - powerful Witches outside a barrier or a Magical Girl actively using magic - she will notice, but not passive ones. If she gets close enough to a Witch inside a Barrier, she can sense where it is, but that's very short range, and it's something she has to work at - she practiced by looking for the 'radiation' emitted when she uses her Clones and Invisibility, but she doesn't look for that unless she expects to have to.