A question about vector port declaration

To make it easy to understand and be absolutely sure, I would say: why won't you just write it down the way you want it to be. So if you want to do

A(15 downto 8)<= B;

A(7 downto 0) <= C;

That's what you write down, if you want it the other way, you write it the other way.

But for the statement itself I would assume it means the first one, but I'm not sure.

To be sure you could always try to put some known values in it and see what happens

*edit*

Oops sorry misread the question, ignore this message, I'm sorry for the inconvenience