Pat - Like you, I'm fascinated by dunes. My understanding was that beach grass was planted along the foredune, which is on the ocean side of an oblique dune. The grass takes hold and thrives in the dune, where other vegetation (including trees or forests) succumbs. The grass catches sands blown by the winds which hinders the development and advancement of the dune (or the oblique dune behind it). You're right about no vegetation growth on an oblique dune, the only stop to it is higher land.