vhdl-entity-file-name

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

(".*" . "\\&")

Documentation:
Specifies how the entity file name is obtained.
The entity file name can be obtained by modifying the entity name (e.g.
attaching or stripping off a substring). The file extension is automatically
taken from the file name of the current buffer.

FROM REGEXP is a regular expression matching the original name:
".*" matches the entire string
"\(...\)" matches a substring
TO STRING specifies the string to be inserted as new name:
"\&" means substitute entire matched text
"\N" means substitute what matched the Nth "\(...\)"
Examples:
".*" "\&" inserts original string
".*" "\&_i" attaches "_i" to original string
"\(.*\)_[io]$" "\1" strips off "_i" or "_o" from original string
".*" "foo" inserts constant string "foo"
".*" "" inserts empty string

You can customize this variable.