De Re Militari was translated into English, French and Bulgarian before the 15th century. Where can I find a scan of the Bulgarian translation?
There isn't one.
The idea that there was a pre-Gutenberg Bulgarian translation of Vegetius' De Re Militaria seems to have started with an unsigned article in the 11th edition (1911) of the Encyclopedia Britannica. The relevant passage is in vol 27, p.968, which states
In manuscript, Vegetius's work had a great vogue from the first and its rules of siegecraft were much studied in the middle ages. It was translated into English, French and even Bulgarian before the invention of printing.
… For a detailed critical estimate of Vegetius's works and influence see Max Jähns, Gesch. der Kriegswissenschaften, i. 109–125.
Looking at the bottom of page 120 of Jähns's book (which, for the record, is the 1889 Geschichte der Kriegswissenschaften, vornehmlich in Deutschland = History of Military Science, principally in Germany; see), we see a thorny thicket of black letter type, the fearsome Fraktur. The relevant passage in cleaned-up OCR Roman type says:
… und eben damals, also in der ersten Frühzeit der italienischen Literatur, wurde sie von dem Florentiner Bono Giamboni in die Vulgärsprache von de Meung ins Französische übersetzt.
which properly translates to
… and just then, in the earliest days of Italian literature, it was translated by the Florentine Bono Giamboni into the vulgar language [meaning 'vulgar language' = 'vernacular'; that is here: Italian; kimchilover],, by de Meung into French.
The Fraktur forms of capital B and V used there are very similar, as can be seen from alphabet charts like this one:
The last line above the footnotes on page 120 shows both letters in "Beschäftigung mit Vegez" [~"study of Vegetius"] and on the line above that one we can see that what is printed is Vulgärsprache [~"vulgar tongue", "vernacular"] and not the non-word Bulgärsprache, which a non-German speaker might reasonably assume to mean "Bulgarian language", even though the German words for Bulgaria and Bulgarian have an ordinary "a", not an umlauted "ä".
In a comment, LangLangC points out that even Google search correctly spell-corrects it to "Vulgärsprache" and only knows about any 'Bulgärsprache' as artifacts in very few badly OCRed fraktur books. He also points out that footnote 5 on that page refers to the Italian title of a translation of a work bound with Giamboni's translation, "Lucano in prosa volgare" [= Lucan in the common language, i.e., in Italian, as also evidenced by the library signature "Bibl Naz II ii 73"], which explains Jähns's choice of the word "Vulgärsprache".
It is clear that the Encyclopedia Britannica article writer misread the V for a B, and thus invented a Bulgarian translation that was really an Italian one. (Giamboni is discussed here; his translation here.)
(click for full view)
Max Jähns: Geschichte der Kriegswissenschaften vornehmlich in Deutschland (Band 1): Altertum, Mittelalter, XV. und XVI. Jahrhundert — München, Leipzig, 1889.
(UB Heidelberg, OCR with a V instead of B but otherwise also low quality OCR)
Many thanks to LangLangC and Denis de Bernardy, for many suggestions and for help editing this post.
POSTSCRIPT: after writing all this, I found L.K. Carley's 1962 University of Nottingham PhD thesis, The Anglo-Norman Vegetius: a thirteenth century translation of the "De re militari", which reaches the same conclusions (but expressed more mildly) about the mythical Bulgarian manuscript on pp.33–34:
I have had no success in tracing the mediaeval Bulgarian translation recorded in the Encyclopaedia Britannica (Eleventh Edition included) and subsequently referred to by Phillips in his Introduction. I am tempted to assume that someone, with the German Gothic print of Jähns before his eyes, has interpreted the phrase "in die Vulgärsprache" (on page 120 of the Geschichte der Kriegswissenschaften, I) as "in die Bulgarsprache", a strange error, if this is indeed the case.
Here the "Introduction" is to the 1940 anthology Roots of Strategy by Thomas R. Phillips; on page 32 Carley comments
The editor, Brig. Gen. Thomas Phillips, contributes a ten-page Introduction which astonishes by its racy style and unprincipled plagiarism, the latter generally in the form of lengthy quotations from Charles Oman and others. Both in the Introduction and translation itself the customary footnotes take the form of gratuitous snippets of information inserted in parentheses in the body of the text.