While browsing the Europeana database here I stumbled over 17th century manuscript Von der Universal Tinctur signed in 1677 by Christian Wilhelm, Baro von Krohnemann. The drawings include few alchemical furnaces shaped very much like the famous Voynich manuscript 'jars'.
16th century German manuscript here has furnaces with VMs-like stands.
It is not that unusual, in my opinion. Not all herbs were consumed raw. The 10th century Dioscorides in the Pierpont Morgan Library (MS 652 here) includes few stoves in its recipes section, so the Voynich Manuscript would not be the first one if some of the 'jars' turn out to be stoves or other heating devices.