I kinda remember that there was one example of blendshape: it was using one of the riggings at obj level. It's about three bones rigging a cylinder.
In that example we can delete non-morphing part of the morph target after we index all the vertices.
And the output of blendshape and skeleton animation still gets a good result.

In kineFX I tried directly using blendshape node to gather morph target and original bind mesh. I find that if I delete non-morphing vertices after I give them same index it will give me a error showing blend shape geometries does not have same structure at bone deform node. I haven't try output it to game engine yet. Has anyone tried only maintaing the moving vertices and still get a correct character morphing result?

I made a mistake of destroying the base mesh for morph. I had to sculpt the 1st and already heavily edited morph target to make it look alike the base mesh. However, since I have to continue sculpting all morph target based on the 1st morph target and unable to only preserve moving vertices. The difference between the true base mesh and the 1st morph target will multiply if I add blend weights to more and more morph targets in the blendshape node.