How do I “fill in” the vertices so that this section of my model will have a face?
It depends on if you care about topology or not. You have to switch into edit mode, select the vertices you desire to be filled and hit the “F” key on your keyboard for fill.
It looks like you’re making a car. The way you’re modeling is not a very good method to begin with.
Hitting F will just fill the area with a giant n-gon. It’s likely that what you want is to strategically grid-fill (ctrl-F>Grid Fill) the open areas in small parts, but this will require a lot of preparation work to get the vertex counts to work. Making a randomly spaced outline as you have and filling it retroactively is really not going to work well though. You need to go back and study proper modeling techniques like box modeling or you’re just going to drive yourself crazy trying to fix something that you’ve already broken.