Dhampir are offspring of one human and one vampire parent, and originate from Balkan folklore.
In an essay entitled "The Vampire" by T. P. Vukanović, contained in the larger work Vampires of the Slavs by Jan L. Perkowski in 1976 and originally in the Journal of the Gypsy Lore Society in 1959. There are a few sections that speak about these children of vampires (see below).
While exact dates aren't given, the author (TPV) discusses the origin of the vampire folklore and gives dates starting in the early 1700s.
Dhampires have long been part of this folklore, but they have largely been shared through oral traditions, rather than by texts.
While other research and evidence on this matter surely exists, this should be sufficient to understand that most cases of vampire offspring in science fiction and fantasy works are derived in some way from these historical beliefs.
Whomever the first author was may be hard to discover, as those oral myths were surely written down long before they were published in a work of fiction. Especially considered that this research paper was published in 1959, predating Changeling by 21 years.