In the Zodiac spreadsheet, Eggy added the MS You are not allowed to view links.
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Login to view., also known as the Bonmont Psalter, which I would like to point out as potentially relevant. The usual caveat: I don't recall discussion of this MS, but this may be classic Voynich amnesia.
Second caveat: this MS is relatively early for our timeframe, made in the middle of the 13th century (Upper Rhine Region). I see it as an interesting node in a network rather than a potential direct source.
The interesting section is in the beginning: the MS begins with a calendar, certainly derived from a different source than the psalter part. I will list some things.
1. "Star" with each Sign
This one should immediately stand out to anyone interested in the Voynich Zodiac sequence. Each of the signs is shown semi-interacting with a big, spiked, faced "star". From the accompanying text, it is immediately clear that this heavenly body represents the Sun: in this case "sol in Leonem". The addition of the sun is thus perfectly logical: each sign is active while the sun passes through it.
Did the VM stars in Pisces, Virgo and Scorpio derive from such tradition? Quite possibly, in my opinion, but then completely bastardized. They no longer stand out as suns, and Pisces has two of them. So if the VM nonsense derived from a "Sun in Sign" tradition, its originally sensible meaning was no longer followed.
2. Some stylistics
As said, the Bonmont Psalter and the VM are almost two centuries apart, and their styles clearly belong to different periods. If there is any connection, I believe we must imagine additional nodes between them. Still, there are some particularities worth pointing out.
First, all suns have full heads inside, a parallel we have had trouble finding in late medieval sources. Those prefer the smiley face: a mouth, nose and eyes, while the VM, early sources and some renaissance artists sometimes complete the sun-face with hair and a jawline. In the latter case, it feels like a personification is imposed on the celestial body. You can see this in Leo posted above, and different faces/angles in the Libra and Sagittarius suns below.
I also noted some possibly coincidental likeness between the style of drawing the outer rays with dotted triangles (central Rosette). And the way the libra sun splits an asterisked field with its twelve red, triangular rays.
3. Taurus "basket"
The main struggle with finding parallels for Taurus is the apparent feeding contraption both bulls are accompanied by. The wicker pattern suggests that the VM artist did see these as mangers of some sort. The green paint seems out of place since I presume mangers would be filled with hay during the winter months. Which brings us to the main reason you won't find any bulls eating from mangers in calendars: they would just go out to pasture. And either way, mangers would not look like this.
(Notice also that the VM artist shoved trees in Aries faces, equally inappropriately; sheep seek out vegetation that grows low to the ground, like grass and herbs).
What if the awkward, beige cylinder was something else entirely in the source? In the Bonmont Psalter, Taurus shares the space with a rare monthly labor, which appears to me like grafting. The root stem has been prepared, and the man appears to be preparing a twig for attachment. So imagine a Taurus in such a grafting (or pruning?) scene: the bull might be close enough to a cylinder to make it look like a weird feeder to the inexperienced artist.
In short, I think this manuscript's calendar has some potential to, given one or two in-between steps and modernization of some signs, explain some of the most bizarre things we see in the VM Zodiac. Or it could all be coincidental.