Here is one of the reasons I wanted to start a post specifically on the head...
Koen has pointed out some good parallels to images on coins and I think it would be difficult to find anything better, but I've been mulling over some ideas that I thought I'd put out there...
The first time I noticed the head I remember wondering whether it might be a self-portrait of the VMS mastermind. It was very common for scribes to insert themselves somewhere in their works, whether it was in an illuminated initial, a colophon, a riddle, or steganographic text (usually near the end). It's also possible that it's mnemonic, but I couldn't think of a really good memory-trigger that might fit both the head and the plant.
I also had other thoughts... maybe the head is not the creator of the manuscript, maybe it's one of the "heroes" of medieval authors (e.g., Galen, Pliny, Aristotle, Plato, Ovid, etc.). Images of classical authors and orators are mostly imaginary portraits, however, and one would have to find a close match to something extant to make a good argument for the VMS profile being one of these historical figures.
In a separate thought, I asked myself, if we had to guess whether the person who conceptualized this manuscript was more likely to be a supporter of the pope or a supporter of the Holy Roman Emperor, which would you guess?
My guess would be Holy Roman Emperor. There is a relative dearth of Christian religious imagery in the VMS but there might be references to the Holy Roman Emperor, such as the Ghibelline merlons, the crown with the cross (the iconic symbol of the HRE at the time), and perhaps a few others.
So... assuming this is a head in the tendril, and taking into consideration the head-wreath implied by the fruit and over-arching shape of the plant, could this head-tendril be a reference to emperors? The iconic image for emperors before the crown-with-a-cross was the head-wreath. Might the image in the plant be a Holy Roman Emperor of the late 13th or early 14th century?
I looked up some imagery of emperors. Unfortunately, many of the paintings of emperors are later re-creations or pure guesses as to how the person looked, but I found that Wenzel of Luxemburg (HRE 1378—1400) and Ruprecht II Wittelsback (1400—1410) were not usually depicted with full beards although there are some portraits of Wenzel from the front with long muttonchops (cheek beards). Ruprecht is usually shown clean-shaven or with a goatee. But I have noticed that later portraits of famous personages often tend to exaggerate the beards.
Charles IV of Luxemburg (1346–1378) is shown with a beard and was also king of Bohemia.
Sigismund of Luxemburg (1433–1437) is also shown with a beard, and was King of Bohemia, King of Italy, and King of Germany, thus briefly uniting a larger territory than his predecessor. When borders are dissolved (even temporarily), there tends to be more movement between regions and greater exchange of cultures, which may or may not have a bearing on the VMS imagery.
The Habsburgs took the crown in 1438 and they were known for their prominent noses and jutting lower lips and jaws, but they are not usually depicted with beards.
Getting back to Sigismund... in many portraits, Sigismund is not shown with a round lumpy nose or distinctive features like those of the Habsburgs, he could easily be Lithuanian, German, or Swedish. There are exceptions, however. There is one portrait in which he is shown with jutting nose and beard but I haven't been able to find the date and I suspect it's a later portrait.
There are some almost-contemporary portraits, however. Sigismund was featured on tarot cards from the mid-15th century in a straight-on pose, with a forked beard. Another emperor (unidentified and possibly iconic) with a long beard is shown in 3/4 view on a card that post-dates the Sigismund image by a few years.
In another tarot deck, Charles VI (King of France, b. 1380–1482) is shown on the emperor card in profile, with a long beard but not a particularly rounded nose. He was known as Charles the Mad and if the VMS plant were something like jimson-weed, henbane, or ergot, one might be able to find a parallel, but the plant looks more like a member of the
Rumex family, or possibly
Bryony. If it's
Bryony, it is poisonous, especially the roots and berries, but I don't think it has a reputation for causing madness (the "Aminean bryony" used in Roman medicine was not the same as toxic Bryonia).
For the most part, the Roman Catholic church discouraged beards (although there were a few popes who thumbed their noses at convention). Beards came into fashion in the Vatican in the early 16th century and died out again at the end of the 17th century.
So, those are a few thoughts that crossed my mind.