If I have in my conf.d Apache directory 001_vhost.conf 002_vhost.conf ... etc, does Putting RewriteEngine On in 004_vhost.conf enable the Rewrite Engine for 005 006 007 008 etc vhosts, but not those that came before in the virtual host load order? This is assuming I don't have that directive anywhere else.
It depends where you have it in the configs (i.e. which section, not which file).
For example, if you have
RewriteEngine on within your VirtuaHost section, that will apply to that host only.
If you put it in the global config, that will apply to the main host (not your virtual hosts which need to be enabled individually).