Hi Jay
VLC at this stage works best with default textures- The specific land classes were chosen for the default texture that shows in NZ. In saying that, any other textures will 'work' but as some users of GEX and UTX have noticed, their textures aren't the best match for NZ AND they install into/over the main/default texture directory which means changes are global.
As mentioned, if you are worried about the effects the textures will have in NZ, then you can try installing the GEX/UTX textures and their landclass in their own directory (i.e. UTXScenery) with two subfolders (Scenery and Texture)- Any included *.bgl files can go in 'Scenery' folder, and their replacement textures (either .bmp or .dds) into the 'texture' folder. This will mean that the textures will only be called by a landclass bgl in that scenery folder rather than the changes being global (and, in NZ, VLC will still use default textures). Im not sure if UTX/GEX actually include replacement landclass so if not, just copy the default worldLC.bgl into the scenery folder you have created.....it may be a bit of fiddling to get what you want

I can't give specific advice unfortunately as I don't have either GEX or UTX .