How can I explain that my human vampire developed venom?

Question

The vampires I have in mind are a sub-species of humans: they share a similar immune system with us, however they evolved fangs and are capable of injecting some kind of substance into their victims, usually a human being.

This chemical not only prevents the blood from clogging, it can also paralyze the prey. When threatened my vampire can alter the potency of the chemical so that it can kill any person within minutes.

How does my vampire acquire the chemical and seemingly not be affected by it?

Answer 1

An old motto in medicine is in dose venenum (the poison is in the dose).

That's exactly what your vampires do: if they inject a low dose of their chemical it simply induces muscular relaxation and prevent movement.

On the other hand, if the injected dose is higher, the relaxation will be too deep and death will result.

That's the reality with any muscle relaxant (ask an anesthesiologist for more info). A simple example is botulinum toxin: on very low doses it is used to relax muscles (as aesthetic treatment), on higher doses it kills you.

As how to explain how they acquired the trait, you can go along the way of snakes: some saliva glands were modified to secrete the chemical, and of course the vampires also developed the counter enzyme to be protected against their own chemicals (the "I just bite my tongue" scenario).

Answer 2

As with any venom, they produce it naturally. The different potency can be explained away by dosage. Normal dosage paralyses; full contents of the venom sacs kill.Have a look at Wikipedia for the different types of venom and delivery. I'd recommend spiders or snakes (or, if you're feeling adventurous, the mammals are at the end).

As with the above, your vampire isn't affected since

...[the venom is] produced by glands below the eye (the mandibular gland) and delivered to the victim through tubular or channeled fangs;

or

...glands that produce this venom are located in the two segments of the chelicerae, and, in most spiders, extend beyond the chelicerae and into the cephalothorax. The fang, the organic functional equivalent to a hypodermic needle is what penetrates the skin, fur, or exoskeleton of the spider's target—spider mouthparts are primarily intended for envenoming a spider's prey in most species, typically insects and other small arthropods. The basal portion includes all or part of the spider's venom glands, which can be squeezed to control the amount of venom forced out of the glands. Such control permits a spider to administer either a dry bite, a dose appropriate to the nature of the prey or enemy, or a maximal dose;

https://en.wikipedia.org/wiki/Chelicerae

or

...[venom is produced in] a venom gland (housed in the maxillary fossae) [passes through] a delivery mechanism of the venom (the maxillary canals), [to] an instrument by which a wound for venom delivery can be inflicted (the ridged canines)

https://en.wikipedia.org/wiki/Euchambersia

Answer 3

The venom can be cause of evolution. Maybe the humans you attack are being so aggresive and its dangerous to hunt because they have weapons, then you develop a strongest body and/or venom to paralyze humans.

Check how Snakes of Ilha da Queimada Grande (snake island) venom evolved because preys they hunt (mainly birds) die outside of the island, due to a slower and less powerful venom. They develop a better venom to kill animals in a shorter amount of time, before they escape from the island.