What follows is my own little academic detective work taken out through use of my university's database. It's a very expansive database, with a wide range especially on historical sources, and it's an AAU, if that helps set your mind at ease about it. I've searched in the two languages that I qualifiably speak, english and French, and would like to note that it would be helpful if there's anyone that's knowledgeable in Ottoman/Modern Turkish, Greek, or Arabic might have access to useful research that I, alas, cannot read. So I welcome anyone else to help broaden my search/correct me if you find anything to the contrary.

So what I first did is try to collect any and all sources that discuss human sacrifice by the Turks. It uses a boolean search system, so I searched for "human" and "sacrifice" with varying combinations of "turk" "oghuz" "murad" "mehmed" "Cantacuzene" "Mongol" "Anatolia" "slave" "Uralic" "Altaic" and "Ural-Altaic". I also translated these same terms into their french counterparts and searched that way as well just to be sure I had what bases I could cover, covered.

After all of This, I turned up 2 sources: Cantacuzene's Contra Mahometem Apologia or Apology Against Muhammad as far as my recollection of High School Latin tells me and Vyronis' essay you've mentioned, specifically the section entitled Evidence on Human Sacrifice Among The Early Ottoman Turks published in the Journal of Asian History Volume 5 published in 1971. Nikephoros Gregoras, whom you mentioned, seems to be a medieval writer who lived from 1293 - 1361 AD -- I couldn't find anything relating to this in whatever body of his works that I could search, though this may be limited by my lack of knowledge in Greek. But as far as I've searched, these sources that you've already talked about are the only ones mentioning this. I'm going to go deeper into the actual sources and their claims if you'd like to read on, if you don't, and that was all the answer you were looking for, that's fine too.

Speros Vryonis himself seems to be a fairly established academic, an american of Greek Descent whose primary works seem to concern Byzantine and Greek history as well as that of the Balkans. He forays in Turkic culture, but only insofar as it relates to Greek-Turkic relations, as evidenced by the title of his big work on the matter The Decline of Medieval Hellenism in Asia Minor published in 1971. He was born in 1928.

His paper cites an instance in which an author referred to as Ducas, writes of an "inhuman custom" that Mehmed I used to mourn his brother Musa, which he then posits must have been well known to the readers of the time if he were to say it in such vague but telling terms. This work by Ducas was entitled Grecu, but I could not find it, only finding that Doukas is the name of a noble Byzantine family. I cannot substantiate of refute this source, but since he just refers to an "inhuman custom" much in the way someone gossips, I don't know if there's anything of great academic fruit that can be wrought from the one quoted line of this source.

Next is Cantacuzene and his Contra Mahometem Apologia. Vryonis in his own work says that it was written "At the bequest of Mellitus, formerly an Ottoman ulema who had deserted the court in Bursa [Ottoman Capital at the Time], come to Constantinople, and converted to Christianity. In this polemical tract, Cantacuzene set out to prove that Islam was not a truly revealed religion." He utilizes, according to Vryonis, "Classical categories of proof" developed in the Byzantine Polemic, one of these was to prove the falseness of Islam by showing it "preaches the sword and murder". I don't know if they mean to discuss forced conversion here, which could be thoroughly debunked through several sources. One would be a source I perused through while doing this, Conversion to Islam in the Balkans by Anton Minkov. He actually mentions Vryonis by name and his "two dimensional" analysis of conversion and Ottoman-Greek Relations. I leave it up to you to take that into consideration. I'll tell you what conclusion I've come to that statement later here in the end paragraph. Anyway, Cantacuzene seems to be making a pretty large accusation: "For whenever a muslims goes to war and one of them falls in battle, they do not blame themselves as causers of the war, but each one slaughters as many living men as he can over the dead body." It's easy to sense a great deal of bias here, and it could easily also be construed as ottoman fighters just being more encouraged to keep fighting when one of their comrades die. Although the next part of the quote goes on to talk about the actual post-battle burial of dead soldiers and how they will try to go and kill captives and if they dont' have any, go off and buy christian slaves to buy and do the burial. The timeline that I would imagine it takes to go from a field of war to a proper burial with sacrificed slaves involved seems to make much more time and is way out of bounds with what actual Islamic burial looks like, and by this time the Turks are very much so Islamized. It is also notable that muslims should be buried preferably hours within their death, not carrying bodies back and then massacring slaves and all to bury them. If you'd like to read more on Islamic burial here's a link. Regardless, considering the reasoning for making this work in the first place, as told by Vryonis, this work is questionable as far as reliability when talking about the Ottomans go. Cantacuzene then goes on to refute his own claim by talking about how this is actually an asiatic tradition, relating to shamanistic belief. I cannot find anything that would link the shamanistic turkic beliefs, Tengriism, with human sacrifice, though admittedly this is not an area of my expertise, so someone is free to correct me here. But the idea of Tengriists sacrificing humans to commemorate the dead is unheard of to me, especially amongst the Anatolian Turks and Oghuz. I can't even locate a source that mentions this, so if anyone does maybe we can delve into it then. So this is all questionable so far -- the claims of Ducas, and the claim of Cantacuzene -- and it leaves one more guy, Chalcocondyles.

He is quite the famous historian who recorded about the last 150 years of Byzantine history in his ten volume Histories (creative, I know). He is the one from whom this story of Murad II buying 600 Greek slaves to sacrifice in the memory of his father comes from. Vryonis tries to tell us afterwards that Chacocondyles is the most unbiased historian of the time and so we need a great deal of evidence to try and refute his claim. This is my attempt: The Oxford Dictionary of Byzantium Gives us some light on the life of Chalcocondyles -- HIs family lived in the Peloponnesus, his father was sent on a diplomatic mission to negotiate the freedom of certain states which ended in his father being imprisoned and Murad going off and wreaking havoc on the Isthmus of Corinth. His life details are also a tad fuzzy, and what he was witness or not witness to is often up for debate. He did supposedly serve as Murad II's secretary, but I'm finding this from an unsubstantiatable source, but this tells you how iffy many of his life details are in english.

Ok, so to conclude, all I seem to find is one source that is sourcing 3 others. One, Ducas, is just an unknown that sounds like he's speaking gossip. Cantacuzene is very personally invested into the history, and this claim of his comes up in a paper designed specifically to smear islam/the Turks (as they seem to be two confounded entities here). And Finally, despite Vryonis telling us the Chalcocondyles is an unbiased source, I'm finding it hard to believe that anyone that is chronicling the time that they live in, where they belong to one of the principle political actors, would have no bias at all. This is especially confounded by the fact that his family and the area he lived in was personally affected by not only the ottomans but Murad II. It's also worth noting that these, and the few other earlier sources that were cited, are coming from people that are writing more exploratory chronicles of a people than unbiased and calculated history -- it's as if we were to take everything Lewis and Clark said about the west to be real (read it if you'd like, there are a couple fantastical elements in it). Many of the writers are Greeks, who were often very directly threaten by Islamic and Turkic invaders, so this would reasonably color their view of their writing of them. This is coupled with the fact that the sources attesting to such a terrible and sensational act is surprisingly limited, not what you would imagine. We should also take into account that Vryonis is greek himself, and that his article was written in 1971. Now this is purely my postulating, so take it as a tad bit of opinion, not as fact: the history of Greece and Turkey have always been contentious and filled with a great deal of bad blood, and you have to take this into account when reading the work of a historian of Greek Descent whose work is invested in Byzantine/Greek/Balkan history, a region with a contentious history with the Turkic world. In addition, this is 7 years before Saïd's Orientalism, and the possibility of the presence of an orientalist perspective that tends to a more 2-dimensional and simplistic view of Islamic culture should be taken into note as well. That's what I personally find here at least, I'll leave you to develop your own opinion on the matter.

The wikipedia entry seems to be drawing from just Vryonis without much other knowledge of the source since he's using typography of the names that I only really saw in Vryonis' work -- just another note for why you shouldn't trust wikipedia too much in the first place.