I don't know who all uses Emacs for VHDL but the vhdl-mode major mode is pretty great for the language. Of course, use whatever makes you the most productive. The following is from the vhdl-mode maintainer, Reto Zimmerman....
Combobulate is a package that adds advanced structured editing and movement to many programming modes in Emacs. Here's how it works, and how it can enrich your editing experience in Emacs.