vhdl-speedbar-cache-file-name

vhdl-speedbar-cache-file-name is a variable defined in `vhdl-mode.el'.
Its value is

".emacs-vhdl-cache-\\1-\\2"

Documentation:
Name of file for saving hierarchy cache.
"\1" is replaced by the project name if a project is specified,
"directory" otherwise. "\2" is replaced by the user name (allows for
different users to have cache files in the same directory). Can also have
an absolute path (i.e. all caches can be stored in one global directory).

You can customize this variable.