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?