Insert the mapgen callback rather than registering it. This lets vmg

build before other mods attempt to (and find nothing there), allowing
them to cooperate. It won't work if someone else uses the same trick, of
course.

init.lua

@@ -87,7 +87,8 @@ if vmg.define("respawn", true) then
 end
 
 -- Call the mapgen function vmg.generate on mapgen.
-minetest.register_on_generated(vmg.generate)
+--  Inserting helps to ensure that vmg operates first.
+table.insert(minetest.registered_on_generateds, 1, vmg.generate)
 
 if vmg.loglevel >= 2 then
 	print("[Valleys Mapgen] Loading nodes ...")