What I would do is install the battlenet launcher. In there you should he able to reset the WoW configs to defaults. If not, you can install retail WoW and use it to set your configs. (Don't need an active account any more - free to play till level 20)
What you set in retail should change what yo have for vanilla. You will get a popup stating your hardware has change asking if you would like to reset.
Reset will go with vanilla settings. Not resetting will go with retail settings. If you have 4k or other modern feature. Keep retail configs.