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