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The devil is in the details. I've seen in the same page of this astronomical tables codex three ways to write 'radix'. First the whole word, then the R with the right glyph of Eva-t and Eva-k and finally in red the R with a crossed line to indicate the x. All in the same page. All with the same intention or meaning?

   If you or someone show me the abbreviation of radix with the glyph identical to that of the VM in a herbarium or a book of any subject, that would be more convincing.

This is completely normal in Latin script. Abbreviations were very flexible and sometimes the same word was abbreviated two or three different ways in the same paragraph, occasionally even in the same sentence.

Often it is done the same way as the Alphonsine tables that you linked. Often the first one is written out in full, the next one is partly abbreviated, and the rest (if there are a lot of them, like in tables) are highly abbreviated. They did the same in Greek manuscripts with the word "Peri" and other common words, especially those that occur at the beginnings of lines or columns.