Ah, performance issues. For panda’s geoMipTerrain, I gave up on updates, and just make it brute force. I use rather small tiles to reduce generation stalls, and I generate the height map data on the GPU in multiple passes spread over several frames. This spreading adds a lot of code complexity, but it well worth it. No threading 
I’m considering adding a third renderer to replace the geoMipTerrain based one with my own code so I can split the generation over a few frames, and optimize it for me usage. I’d do this in Cython for full compiled speed. I think there is a simpler height map class I should try first though.
The shader semantics tell the GPU which register to store the value in.
Ex:
float4 vTexCoords : TEXCOORD0;
float4 something : COLOR2;
just choose one, I usually use TEXCOORD# or COLOR# for random stuff.