vhdl-forbidden-syntax

vhdl-forbidden-syntax is a variable defined in `vhdl-mode.el'.
Its value is

""

Documentation:
Syntax of forbidden words to be highlighted.
If option `vhdl-highlight-forbidden-words' is non-nil, words with this
syntax are highlighted in a warning color to indicate not to use them.
Can be used to highlight too long identifiers (e.g. "\w\w\w\w\w\w\w\w\w\w+"
highlights identifiers with 10 or more characters).

NOTE: Activate the new setting in a VHDL buffer by re-fontifying it (menu
entry "Fontify Buffer").

You can customize this variable.