There is another reason vetalas were so important. Sexual practice (“karmamudra”) was central in Indian Tantra. It was said to be the swiftest and surest Tantric method, for those capable of it. A difficulty was finding someone willing to do engage in it with you—since the early forms of karmamudra were considered utterly disgusting. The usual approach, for men, was to hire a low-caste prostitute. Not everyone was able to do this, due to lack of funds or other practical problems. If you were a monk, there were also tiresome issues of vinaya—the monastic vows, which prohibit sex with women.
An alternative was to raise a vetali (female vetala). At a certain point, an official ruling was made that for a monk to practice karmamudra with a vetali was not a violation of vinaya. This made the practice extremely popular. Or, at any rate, there was considerable demand for information on how to raise vetalis.
Authentic sexual practice necessarily benefits both parties. You wouldn’t know that from reading some of the male-oriented literature on the topic; but any attempt to “use” a prostitute, slave, or corpse for karmamudra, in a one-sided way, is entirely self-defeating. Clueful Tantrikas understand this, and practice accordingly. Vetalas and vetalis are often malevolent—like humans—but can be transformed by Tantric practice—just as we can. All sentient beings have Buddha-nature; the undead are no exception. Mainly due to karmamudra, it seems that there were many highly-realized vetalis in India at the time of the Mahasiddhas. No doubt some were famed as teachers as well as consorts.
In fact, at the dawn of Tantra, at least one vetali became fully enlightened in this way. She is Vajra Vetali, Queen of the Vampires, who attained Buddhahood as the consort of Yamantaka. Chögyam Trungpa Rinpoche wrote a hair-raising invocation of her that is regularly practiced in his Shambhala centers: “You enjoy drinking the blood of ego . . . As night falls, you cut the aortas of the perverters of the teachings.”Source