A letter of a formal language is a more or less arbitrary symbol whose interpretation depends on the specific context.
In building a formal language, letters are considered to be the undefined terms of said language.
An important part of assigning semantics to a formal language is to provide an interpretation for its letters.
The vocabulary of a formal language is the collection of all of its letters.
As such, it forms part of the alphabet of this language.
Each signature for the language may have a different vocabulary.
However, different signatures only have different members of the respective classes of symbols comprising the alphabet, rather than introducing new such classes.
Compare with signs, which also form part of the alphabet.