Minecraft version: 1.20.1
Forge version: 47.3.22
Vertical Slab Compat version: 1.20-2.5
(Environmental version: 4.0.1, in case it's relevant)

Grass Thatch Vertical Slab and Duckweed Thatch Vertical Slab seem to use the wrong texture compared to the original from Environmental. I presume something to do with the slightly different model those blocks have.

Can confirm the same issue on my end, although my vertical slabs are slightly different. This is on v_slab_compat-1.20-2.5, environmental-1.20.1-4.0.1, and Quark-4.0-461.

The other thatch slabs from Environmental are generated correctly, albeit without "hairs", which is just a feature this mod doesn't have. These two seems to be using the "hairs" as textures instead of the block sides for some reason.