This is Book Six in the Immortal Knight Chronicles by Dan Davis. I only picked this up because of the Balkan setting, but the previous books all seem to have a medieval setting. I would hesitate to describe this as historical fiction as it has a significant element of fantasy.
Having said that the author keeps pretty close to the historical timeline. Our 'hero' (Richard) is a vampire who has a small band of adherents or what would be called Strigoi in Wallachia. The theme that appears to tie the series together is a conflict with his vampire brother, who in this book fights for the Ottomans and even becomes Grand Vizier.
The historical timeline starts with the Battle of Varna, the failed Balkan crusade. He then goes on to support Vlad the Impaler, who is himself Strigoi, in both of his periods as the ruler of Wallachia. In between, he appears at the Second Battle of Kosovo in 1448 and the Siege of Constantinople in 1453.
His reason for being at these events is as an English mercenary commander. He fights for Hunyadi at the Siege of Belgrade before going back to Wallachia and supporting Vlad in his defence of Wallachia against the Ottoman invasion. This culminates in the Night Attack at Targoviste in 1462. When Radu siezes the Wallachian throne, Richard goes to Moldavia and fights at the Battle of Vaslui in 1475.
Despite mixed success on the battlefield he triumphs over his brother, although this appears a temporary setback as they are scheduled to return to the frau next during the Spanish Armada. Vampires live a long time, so long as they avoid the stake through the heart!
If this all sounds really silly, well you do have to suspend reality - it is after all fantasy. Having said that it is pretty well written and ties in well with actual historical events. I quite enjoyed it, although I doubt I will bother with the rest of the series. You can only take so much blood!
My wargame club, GDWS, did a series of display games based on this period, including Vaslui in 2005. I am still picking the teddy bear stufficg we used for the fog from my trees There was also a great WAB supplement for this period. We had a few tongue in cheek fantasy bits including the Sultan on a flying carpet and the odd Bram Stoker reference. It's hard not to!