With the Eden Editor the ability to place empty versions of vehicles only applies to vehicles which are actually crewed and then with a key you can place an empty version. However many mod developers (especially those which don't make spawn-able crewed units) don't have all their vehicle versions spawn-able with a crew, this means those vehicles cannot be spawned normally in the editor.
A good example is the RHS mod which has several variations of vehicles like humvee's but some are only in the empty section in the normal 2d editor and don't have crewed variants.
This is where this report partially turns into a feature request also as it would be great to have the empty section from the 2d editor put into Eden's so we could access all these vehicles which are currently unnacessible.