Geomorphing terrain with a vertex shader (custom vertex data maybe?)

Hello everyone, I'm trying to get geomorphing working (better) in my planetary terrain engine. Geomorphing is where you morph from a lower resolution mesh to a higher one, in this case to reduce "popping" you see in the terrain. I want to be able to do it in the vertex shader, by passing a couple floats, rather then sending new mesh data every frame.

What I currently have is the main terrain stored in vertices, colors, and normals of the mesh, and then the collapsed vertices are stored in mesh.tangents. I then lerp between them based on a float value passed to the vertex shader. However, I soon realized that I also need to store the collapsed colors and normals as well. I could use the two texture coordinate's but I still wouldn't have enough room.

So I'm wondering if anyone has any ideas on how better pack data into the mesh class somehow, or if there's a way to send custom data to the vertex shader (didn't seem possible with Unity meshes), or really just a different/better way to do this. This is a pretty specific subject it seems, but hopefully some shader experts can help me out, Thanks!