Lord Harkon is the leader of the vampires and is a vampire lord and Mirrak is Dragonborn and has several shouts including one to tame dragons if these two were to clash in battle who do you think would win?
I believe all Dragonborn have the potential to become the most powerful mortal beings in the ES universe. Their potential remains dormant and untouched when a Dragonborn lives the life of an Emperor, but if they embrace their gifts like Miraak did, they become a living paragon of what power really is. Mortal beings with the full power and skill of the immortal Dovah.
Miraak would toy with Harkon rendering what little resistance Harkon could put up as absolutely useless, Harkon wouldn't even challenge Miraak whereas Miraak would pistolwhip Harkon. Harkon would be Miraak's play thing. 10 Harkons wouldn't even touch Miraak.