I think you're too hung up on the interpretation that Dracula from the original novel was supposed to be Vlad Tepes. Van Helsing calls him "that Viode who fought against the Turks", but later in the novel, his past is described as "that other of his race" who lived "in a later age". Meaning, the Vlad Tepes backstory was just one of many possibilities, which is a common trope. "Some say he is X, but others claim he is actually Y", etc. This fits perfectly with Dracula not having a set in stone backstory in media that revolves around him. It's part of the character's mystique and what makes him interesting.