Vertex Color data (outputed by the Vertex Color node in the Material Graph as a Vec4) is just the RGB values equivalent to the ones you used when vertex painting the model in your DCC. If you want coordinates or more vert data there are some WorldPosition and VertexNormal nodes you can use. In the example image I am pulling the Red channel (equivalent to anything up to 255,0,0 vert paint on the mesh) to use as a mask for the texture in the Blue channel of that Param2D node).
Oh, don’t forget to change the Vertex Color Import Option in the FBX importer when bringing in your meshes as it’s set to “Ignore” by default.
