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.