I am making a mobile vr game.  It will be in lowpoly style, and I am trying to find the most computationally inexpensive lighting model that has lighting.  I have heard of PBR, lambertian, half lambertian, and phong, but I don’t know the performance of each of them.

Lambertian diffuse is about as cheap as you can get (the BRDF is effectively a constant). So I would start with that if you really want to go cheap. Later on you can see about adding in some simple Blinn-Phong specular if you have the headroom for it.

If the light is dynamic then dot produxt of fragment normal and light to fragment pos, (not including shadows) (somewhat phong but witout use of exp)

For static nonchangeable light use lightmaps.... Anyway both will give you poor performance when managing big screen resolutions

How would you actually create a lambertian diffuse per vertex diffuse shader for unity?  I have decent shader knowledge.

I like the catlike coding examples for custom shaders in unity.

Custom Pipeline

Custom Shaders

Lights

this is one of those times I don't go to the official docs. Really clear process explanations.