Diane O'Donovan posted some thoughts about the letters in the petals of the violet on fol. 9v of the Voynich Manuscript. She believes the letters "should be Hebrew or another closely related script". Read her article here.
I can't comment on the script. However, I found image that is really close to the 'lettered' VMs violet. It is from the British Royal Collection and it is the Leonardo da Vinci drawing of the anatomy of the human anus.
For the record, I don't believe Leonardo is the author of the Voynich manuscript. I think the above similarity is just a hilarious coincidence.