vhdl-indent-syntax-based

vhdl-indent-syntax-based is a variable defined in `vhdl-mode.el'.
Its value is

t

Documentation:
Non-nil means indent lines of code based on their syntactic context.
Otherwise, a line is indented like the previous nonblank line. This can be
useful in large files where syntax-based indentation gets very slow.

You can customize this variable.