D&D Monsters: Demons

Given that "demon" is such a broad term in real-world mythology and religion, there are inevitably many different interpretations and forms of the beings. They were included in D&D right from 0E, where they are listed simply by a numbered type, increasing in power from one to six. Alternative names are provided in 1E, which become the standard names from 2E onwards. Notably, 2E also tried to pretend that they weren't really "demons", lest that offend anyone religious, and described them instead using the invented word tanar'ri. 3E switched back to "demon" again, explaining that the "tanar'ri" were merely a common subtype; the latter term has largely been deprecated since. 5E restored the numbered type sequence, but only as rarely-used categories of increasing power so that, for example, chasme mosquito-demons are now another example of Type 2 demons, alongside the original hezrou.

D&D Monsters: Vrocks

The word "demon" comes from Ancient Greek where it was used to describe semi-divine beings and generally lacked the negative connotations of the modern word. Early Christians later co-opted it as a term for "servitors of foreign/hostile deities", from which it rapidly morphed into something more explicitly evil. In that sense, it's a common concept in many real-world religions, a generic term not linked to any one in particular and that therefore fits with the typically polytheistic worldview of D&D.

The vrock specifically has no direct antecedents in mythology or in Christian demonology. It may be partly inspired by the demonic Tash from C.S. Lewis's Narnia books, but the resemblance isn't that strong and could be coincidental. Vultures appear in the myths of both African and American cultures since similar birds live on both continents. (In reality, the "vultures" on either side of the Atlantic are not closely related, despite their physical similarity, but this may not be relevant in a fantasy world).