Hi again:
Successfully configuring 3rd party / custom Land Class (aka "LC") scenery BGLs for specific display priority is sometimes complex, and requires repeated testing after considering some general guidelines.
First, be aware that even in FSX / P3D, AFAIK, custom photo-real scenery is still technically another form of custom LC which IIRC, also uses binary Region codes within the FS2Kx / FSX / P3D BGL ...but with an 'alternate' alphanumeric texture tile file name series (inside the BGL in the case of FSX / P3D) that is slightly
different from the default FS LC texture file names mapped to FS terrain grid tiles.
So even when they are added to the FS Scenery Library GUI stack of layers, one may need to consider location / position of a 3rd party / custom Land Class (aka "LC") scenery BGLs within that stack of layers to control "load order".
FYI: 3rd party / custom add-on BGLs are sometimes placed into:
*
[FS install path]\Addon Scenery\Scenery
...or:
[FS install path]\Scenery\World\Scenery
...for special purposes which require those BGLs to load early in the FS startup process, thus loading
"earlier" than non-default scenery BGLs.
In the case of 3rd party / custom Land Class (aka "LC") scenery BGLs, one usually needs to load these files
"later" than default scenery BGLs and certain other non-default scenery BGLs.
Thus, one may (..
always ? 
...) wish to load 3rd party / custom Land Class from layers physically "above"- and separate from- the default
FS Addon Scenery position in the FS Scenery Library GUI stack of layers. 
Also, in some cases the local LC instructions of a BGL may be over-ridden by the slope of terrain mesh "shape" in an area when standard-type LC (either default or 3rd party add-on) is used rather than custom photo-real scenery.
Generally-speaking, one should
not put standard-type LC (either default or 3rd party add-on) or custom photo-real BGLs in:
*
[FS install path]\Addon Scenery\Scenery
...or:
[FS install path]\Scenery\World\Scenery
...and should instead put them into a newly-created \Scenery sub-folder under a top-folder, then added to the FS Scenery Library GUI stack of layers as an 'Active' layer
separate from any layer containing
custom photo-real scenery for the same project area, as discussed in "Scenery Priority" on pages 5 and 6 in this add-on product documentation PDF:
http://www.vectorlandclass.co.nz/downloads/VLC_Manual.pdf
BTW: In FS2Kx, it was necessary as well to
NOT pair the \Scenery sub-folder containing a 3rd party add-on LC BGL with a \Texture sub-folder
unless the mapped land class-type textures for that BGL were
physically present within that local paired \Texture sub-folder, or FS would crash / OOM during extended file searches for mapped textures; reportedly FSX P3D no longer has that problem.
NOTE: Additional considerations may be necessary as well:
https://www.google.com/#q=FSX+Land+Class+priority
Hope this additional info helps !
GaryGB
