Also calls into question how blood "works" for vampires? We've seen vampires consume human food - N serves themselves a cup of tea, so it might just be human liquid. Does this mean that consuming blood merely enables a vampire's digestive system, and maybe the other systems as well, to process the nutrients in human food and drink? Taking a critical look at human blood, it has no nutritional value and consuming too much will cause a human to vomit the blood, violently. It's not a good sign, signaling internal trauma; you can read more here, here, and here.
Agency might supply their vampire agents with blood, or maybe created a "synthetic blood" to get them by if there's a blood shortage. I have no idea how to apply biology to the concept of synthetic blood, though.
Light bulb moment! Could certain vampires develop allergies to blood types? It's an interesting concept!
Venom doesn't explain how vampires turn their victims into vampires. Venom is a toxin which is meant to inhibit an organism. Venom is poison that is injected into an organism in some manner or another i.e. fangs, spikes, or stingers. Snake venom contains proteins, or maybe it's enzymes (I can't remember), that inhibit an organisms natural proteins/enzymes, for example.
For example, if someone gets bit by a poisonous snake, that venom will spread through the body, inhibiting the body's own signals - remember the purpose of venom is to a. neutralize prey and b. help the animal escape a predator (it's uncommon for a snake to use it's venom on a predator, typically resulting in a "dry bite" because the mere threat of being poisoned is enough to deter most predators) - and may cause the individual's muscles to spasm uncontrollably because that venom is firing that signal at a rate faster than normal.
Also, don't get me started on biology and the reconfiguration that the human face - let alone the jaw - would have to undergo if a human-turned-vampire grew fangs and the venom sacks to deliver their bite.
This is why I prefer the explanation of a virus for vampirism. It provides a more believable explanation for the face change and how one could transform into a vampire compared to the venom route.
A vampire bite causing pleasure, or at least numbness, would probably be a requirement. A vampire's fangs are basically hallow curved needles that are sinking maybe 2 or 3 inches, at most, into the victim's neck - ouch. You'd need something to keep your prey docile and not fight back and injure you. Snake venom is primarily used to subdue dangerous prey so that the snake can swallow it without a risk of losing an eye. If a vampire bite, much less the vampire, didn't have a way to keep a human docile, the bite would trigger the fight or flight mode in a human and it's be harder to subdue their prey in the first place. This would be extra risky if you have a vampire hunter, posing as prey, because they're perfectly aware as you munch on their neck - close enough for any action to be fatal. Also, you'd need a coagulant otherwise you'd kill your prey, if you didn't intend to kill them in the first place because you just punctured two 2 or 3 inch wounds into their neck, possibly hitting an artery.
Then again, we can go to as many boundaries as possible because it's fiction!