Mame reads ini files in a certain order (assuming readconfig is left at default enabled).
First mame reads mame.ini, then vector.ini if a vector game, then
sourcefile.ini, then parent.ini then game.ini, and finally the commonline options, each overriding the prior settings.
(Actually, even the above is simplified for easier understanding and less typing
.)
Each of the ini files can be blank, have a few options, or all the options that are in mame.ini. Personally, I like keeping the specific ini files as small as possible; I only put the options I know are different than mame.ini or that I don't want mame.ini to change. This way, if I want to make a global change, I only have to edit mame.ini (assuming the option I'm changing isn't set in any of the specific ini files).
FWIW, if you have mame.ini, vector.ini, and asteroid.ini, and all are "complete ini files" with all the options, mame will act as if only the asteroid.ini file existed for asteroid, and as if only vector.ini existed for other vector games.