The identification of the Voynich Manuscript mysterious plants may be further complicated by the fact that even 'mainstream' medieval herbals contained imaginary herbs. Great example can be found in few Pseudo-Apuleius manuscripts from 11th-12th century containing image of the herb Basilisca (Bodleian Library, Ashmole 1431 here and Ashmole 1462 here, British Library Harley 1585 here ).
The problem with Basilisca is that it only appears next to Biblical Basilisk creature, which is reportedly capable of killing people by just looking at them. To get this herb a person has to be not just a snake whisperer, but a Biblical snake whisperer! How did anybody lived to tell the tale about this plant and give us its description is a mystery. Nevertheless, the portrait of Basilisca exists in the Oxford herbals and somewhat resembles the Voynich manuscript f3r.
The plant portraits in the old herbals often lack realistic details. Another Voynichese image from Pseudo-Apuleius is the heliotrope. Without the author's stated intentions it would be hard to connect it to real plant.