Not sure which sim you have, so this is kind of vague...
It doesn't really make much sense to install VLC, as the only way that Orbx NZ can work means it will over-ride everything. Well, you can install it, but you won't serve any purpose with FTX NZ installed.
I'm not sure about the airfields -- worth a try. I have them working, but really I'm too old to remember what if anything needed to be tweaked. Plus, now that I think of it, I don't have any FTX scenery in P3Dv4, which is the only one I use these days, only the North Island in v3, and both islands in FSX for testing.
GZ Ashburton should work ok, although it was never tweaked for Prepar3d, so there may be quirks. Just make sure that you set the FTX insertion point in FTX Central, so that FTX is below (in the FTX Central list) any Godzone/Real NZ or other custom NZ scenery. If it doesn't get rid of the FTX version completely, let me know, I think I have a tweaked exclusion somewhere.