Alan Peterkin, author of 1000 Beards: A Cultural History of Facial Hair explains in his book:
In the 15th century, beards were once again routinely worn by nobelmen and elders to signify importance, dignity, and advanced age. They were curled with lead iron, parted at the chin, and plastered into submission.
If the Guy with the Beard was in the herbal section of the Voynich manuscript it would be an easy match to the famous Mandarke Man. The split facial hair, however, is in the 'astrological' section and may represent about anybody or anything.
The most famous split beard of the time of the Voynich manuscript parchment is probably the one of Holy Roman Emperor Sigismund as shown on his coins, in Chronik des Konstanzer Konzils and the Albrecht Durer's painting.