Hello, I'm a long time collector of Iron Crosses of all confllicts and glad to make your aquaintance here. Getting my head around the forum world so bear with me!
I'm only going on what I'd always assumed, but my thoughts are that awarded crosses were always flat due to perhaps cost of production and uniformity of awards. I'd never considered award pieces being vaulted but there's really no reason why that should be exclusively the case to my knowledge.
There's nothing I've ever found that confirms this but it would seem to make sense to me.
As to private purchase pieces it seems plausible that you would have a choice of pieces of varying value, whether higher silver content, vaulted or quality of finish.
Just my thoughts on the subject.
Hope to enjoy your company here and add something to the forum in my limited way.
Cheers,
Clive.