vhdl-argument-list-indent

vhdl-argument-list-indent is a variable defined in `vhdl-mode.el'.
Its value is

nil

Documentation:
Non-nil means indent argument lists relative to opening parenthesis.
That is, argument, association, and port lists start on the same line as the
opening parenthesis and subsequent lines are indented accordingly.
Otherwise, lists start on a new line and are indented as normal code.

You can customize this variable.