Is there any way to do a Clip() in the vertex shader rather than fragment shader?

I’m looking for a way to ignore whole polygons in shader… rather than having to do fragment shader Clip() checks, or rendering 0 alpha
Is there an equivalent Clip() type command I can run on some vertex data ?

There is no trivial way. The only way I can think of is to set some value to 1 to clip, 0 otherwise, and do a test in the fragment shader if (val > 0) discard;)

The issue here is that vertex shaders are executed per-vertex, not per-polygon. So the result of dropping vertices, even if it were possible, would depend on the topology of your mesh. You would probably find multiple triangles sharing that vertex would vanish.

Though I have not used them myself, you probably want to look into using geometry shaders.