Brush textures in TF2 are made up of a vmt (material) and vtf (image) combination. Every valid brush texture has a vmt/material, which at the minimum, defines the image that makes up the texture and what kind of "material" it is. The material defines things like what kind of decals and sounds the texture makes, special properties like having different friction, and for blending textures, what textures blend together with what mask. It also states what detail textures, like grass blades, the texture has. You want to edit the vmt, not the vtf. The vtf only contains image data.