I think you're underestimating the amount of effort it would take to rewrite history without anyone finding out about it for centuries. Like I mentioned before, historians are very good at separating the myths and stories from things that actually happened. Taking your example about Vlad Tepes, just imagine all the documents you would have to forge (flawlessly) to make that work. You wouldn't just need wro write up documents that would cover his entire life, but also go forward and backwards in time to place this completely made up historical figure into a genealogy. Secondly, you would also have to fake documents that detail his interactions with other, presumably real, historical figures (or if not also put effort into inventing them). Such an unfathomably large undertaking is bound to contain its fair share of slip-ups ("Hmm, this document claims to be from the 14th century but the language usage doesn't match up with that time period", etc)
I'm not saying my own idea is the only thing that makes sense (I'm sure it contains it fair share of flaws) but I think it's important to realize that making a believable masquerade is actually quite difficult. I think there are strong indicators knowledge about Dracula and the Belmonts is quite well-known in Wallachia. In that case, the only way you could possibly hope to prevent the further spread of this information is to contain the area. Considering Wallachia's size in real life, this would be problematic as well. Howver, considering Castlevania's casual attiude towards historical facts, you could always just say Wallachia in the Castlevania universe is considerably smaller than it is in reality. I think that explanation works best with Castlevania's depiction (in my opinion, at least) of Wallachia as a gothic-horror fantasy land.