The Church needs to be REALLY FRIKKIN POWERFUL to manipulate history in such a grand way
ThisactuallyhappenedIRLthough
Anyway, couple the aforementioned bits about Mathias chilling for 400 years and CoD stating he protected the land, and you have a pretty easy connection to Vlad III. That being said, there are some glaring inconsistencies.
For one Vlad fought
for the Church, as the Order of the Dragon was a Christian sect. Mathias is pretty anti-God, so this wouldn't line up. One COULD theoretically argue him as pretending to fight for the Church's cause against the Ottomans, but given how vehement his hatred of God is in LoI, I don't see much feasibility there.
Secondly, there's the whole "head chopped off and displayed on a stake in Constantinople to prove he was dead" thing, but this can be easily fixed with the next bit.
Let's say Mathias did adopt the moniker of
Wladislaus Dragwlya (which is how Vlad signed his name historically), and usurped the throne from Vladislav II as it's historically writ. He basically uses that period to grow his powers and knowledge of the black arts, and uses the neighboring countries' people as guinea pigs. At some point the Turks do something to piss him off (or perhaps they had something he wanted and they denied him) and he goes after them as we know Vlad III to have done. However, he still relied on the easier-access Wallachian people for his experiments and magical tests.
Eventually, he surmises that the area is no longer useful to him, so he creates a doppleganger through his magic and lets that thing take his place on the executioner's block. He then goes underground and the series canon continues as normal.
The Church later alters records and makes it so that Mathias was the son of Vlad II rather than a random usurper, fought on behalf of the Church and God (to rationalize the decimation and brutality towards the Turks, as well as demonize the Turks and shift blame to them as the cause of the violence against the Wallachian people), and was killed by his enemies after running out of money to pay his mercenaries.
Basically, the Church keeps most of the truth intact, but alters certain key details which rewrite Mathias into a warrior of the Church, so that his actions could be justified as acts in the name of God; in addition, rewriting things so that there wasn't an immensely powerful sorcerer and vampire running an entire country, but a member of a well-known Christian order who was the tragic victim of political kidnapping and murdering of his family, who rose up in righteous vengeance against his enemies and died fighting them.