Some background...
Almost all medieval ciphers consist of substitution or steganography or both (it's hard to find examples that don't use one or either or both of these methods). Most of them were not very sophisticated. Many of them substitute one-to-one or substitute only the vowels.
Specifics about the Italian Diplomatic Ciphers collected by Tranchedino
The diplomatic ciphers collected by Tranchedino (about 170 are recorded in a print facsimile) are not very sophisticated and they generally use the same game plan (the glyphs change but the general format does not). They relied on a large library of glyph shapes, and many-to-one substitution, but linguistic patterns can still be discerned in many-to-one ciphers. The Friedmans and the WWII Work Group would have no trouble cracking this kind of cipher.
The main characteristics of the Italian diplomatic ciphers are as follows:
- The substitution glyphs are mostly based on Latin letters, abbreviations, or symbols, or slight modifications of these. There are also a number of arithmetic, geometric, and Greek symbols.
- When additional symbols were needed, they often doubled a symbol (which made the cipher appear more verbose) or made slight modifications to a Latin symbol by adding a tickmark or stem.
- A few of the ciphers substituted number symbols rather than letter/abbreviation symbols (the numbers were mixed with textual nulls).
- There are several optional substitutions for each letter of the alphabet (usually 2 or 3 options but occasionally 4). Thus, g, o, or p might stand for "a". Sometimes biglyphs and occasionally triglyphs are substituted for a single letter of the alphabet.
- There are usually a handful of nulls (most of the ciphers have between 8 and 24 nulls). The nulls are usually Latin numbers and abbreviation symbols, presumably so that they blend well with the Latin letters. There are also Greek abbreviation symbols, but they didn't use these as frequently as the Latin
- Each cipher includes a table in which common names and words are substituted by one or two symbols. A common word like "Papa" (for pope) was generally substituted by one or two letters, although sometimes a whole word (like "quare") was used. There were usually about 15 to 40 words included in this table.
To sum it up, they are many-to-one substitution ciphers (and occasionally also one to many) with a few nulls and a lookup glossary for common words.
If you want to see an illustration of the format, here is one I posted as an example on a 2016 blog:
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The VMS might be a substitution cipher. It might be steganography. It might be both.
If the VMS uses substitution, then it is typical for its time in the broadest sense and the similarity to diplomatic ciphers may only be through the mechanism of substitution, which was how almost every cipher was constructed at the time.
If it is believed that the VMS is a substitution cipher, I think it would be more productive and less confusing to call the VMS a substitution cipher, or a verbose substitution cipher rather than an "atypical diplomatic cipher". Otherwise almost all medieval ciphers could be classified as "atypical diplomatic ciphers" and that isn't very useful since each one has to be solved in its own way and many of them have no diplomatic content.