Mirroring to create a single polygon across the mirroring plane I'd like to create a single triangle by mirroring one vertex exactly on the mirror plane and another vertex across the mirroring plane to create a single face. I've shown an example image. In this example, I've already applied the mirror modifier but it won't create a face across the mirror boundary.



I'm really hoping something like this is possible? I guess I could just leave this one triangle until the very end, then apply the mirror modifier and fill it in, but it would be nice if I could leave the mirror modifier in place and have it correctly construct this face. Is there a trick to doing this?
The mirror modifier in your illustration is working exactly as it is designed to do. In this case, it is mirroring an edge. To complete a face, you need to add one more vertex, located at the intersection of a line perpendicular to the y axis, and the y axis, which passes through the vertex furthest from the y axis, and create the face. The easiest way to create this vertex in the illustrated figure is to extrude from the vertex at the far right to the y axis, constraining the extrusion to the x direction. If you do this, the mirror modifier will work the way you wish.