vhdl-underscore-is-part-of-word

vhdl-underscore-is-part-of-word is a variable defined in `vhdl-mode.el'.
Its value is

nil

• This variable is obsolete since 24.4;
  use `superword-mode' instead.

Documentation:
Non-nil means consider the underscore character `_' as part of word.
An identifier containing underscores is then treated as a single word in
select and move operations. All parts of an identifier separated by underscore
are treated as single words otherwise.

You can customize this variable.