How quickly did the VMS leave the possession of those who understood it?

[Jorge_Stolfi]

Did the person who added the month names understand the MS?

I don't know of any solid evidence for either answer.  

However, the sign icons were almost surely drawn by the same artist who drew the rest of the diagrams and text, in the 1400s.  Thus whoever wrote the month names did not need to understand the text.  And aren't the letter forms of the month names from centuries later?

All the best, --stolfi