In the Curse of Strahd adventure, in Amber Temple, area X42, the following is described (p. 196):
West Sarcophagus. The vestige within this sarcophagus offers "the dark gift of the Vampyr" to any humanoid creature of evil alignment that touches it. The Vampyr's gift is the immortality of undeath.
Reading on, this vestige grants any creature who accepts its gift the option of becoming a vampire if its conditions are met. Clearly this is the entity referred to in the opening description of the Amber Temple chapter (p. 181):
Strahd communed with these evil vestiges and forged a pact with them. When Strahd later murdered his brother Sergei, that pact was sealed with blood. Strahd transformed into a vampire, and the Dark Powers turned his land into a prison.
Although it mentioned "evil vestiges" (plural), the one who transformed him into a vampire must have been The Vampyr. But beyond its name, and the fact that this entity can transform people into vampires via a very specific method, the adventure does not further expand on The Vampyr. I've found basically nothing regarding lore on The Vampyr online, so I am at a loss learning more about it.
The reason I assume this to be an entity is that, based on the other gifts (at the very least, those in X42), each gift seems to be from an entity, such as Zhudon or Tenebrous.
Is there any official lore on The Vampyr? I'll happily accept lore from earlier editions of D&D if there is no further mentioning of The Vampyr in D&D 5e. It might also be worth mentioning that this adventure is the only thing I know regarding Strahd/Ravenloft.
Best Answer
In the original 1983 module I6 Ravenloft, the players can find the Tome of Strahd which the party finds largely unreadable except for the following passage (pg 32):
While Strahd refers to himself as "the Vampyr," the entity with which he makes his pact is called "Death."
Subsequent revisions to the original 1983 campaign did not change the Tome's text (pg 62 of RM4 House of Strahd [1993], pg 22 of Expedition to Castle Ravenloft [2006])
In the novel I, Strahd the entity that gives Strahd the gift of vampirism is also called a manifestation of Death itself:
Of course, Strahd is the narrator of I, Strahd and it is entirely possible that he is referring to the entity as "Death" incorrectly (or as shorthand) when its real name is, in fact "Vampyr."
The fact that he calls himself by this name may suggest that he has taken on not just its aspects but also its name and identity.
It would be up to a DM to assume this interpretation, though (but, given the evil nature of Strahd, it's not unreasonable to assume that he is an unreliable narrator or simply can't reveal information that he himself is unaware of).
The Amber Temple and dark vestiges only exist in the 5th edition campaign and did not feature in earlier modules/campaigns. However, the 2006 update Expedition to Castle Ravenloft expanded the setting beyond the castle and town of Barovia to the land of the same name. In that version, Strahd draws his powers from three "fanes" (witches) in Barovia: Forest, Swamp, and Mountain.
The Mountain Fane, Baba Zelenna, and her lair (a tower) are the closest thing to a predecessor of the Amber Temple and dark vestiges.
In short, while Strahd has historically refers to himself as "the Vampyr," the entity is said to be "Death." Fifth Edition's use of the Amber Temple and its dark vestiges, including the Vampyr entity, is sort of a "remix" of names, entities, and places from the pre-existing lore.
Therefore, because of the ambiguous nature of the Vampyr vestige and Strahd's relationship to it, the DM has a lot of leeway to play with it for their particular campaign.