The problem is that FSDT KLAS does not completely overlay that photoreal layer as airports normally do, for example FSDT KLAX does it fine with Megascenery California.
That's not the problem. As I've said in one of my previous posts, KLAS doesn't overlay a photoreal scenery, because it comes with its own photoreal scenery so, if it was made to cover up a photoreal scenery underneath, it would end up covering its own photoreal scenery too.
So, the real issue is not how the KLAS AFCAD is made (it *must* be made that way, just like any other airport that comes with a photoreal scenery in the background), but why the Aerosoft photoreal is "winning" against the KLAS photoreal, even if it's on a lower layer.
Are you sure it's not coming with a mesh too, maybe one that is of a (nominally, not real) higher resolution than FSDT KLAS mesh ? If that scenery comes with a mesh .BGL and it's a separate file just for the mesh, a possible solution might be removing just that file.