You know what you want, it's on the way, no worries. Use the intervening time to start picking up magazines [between eight and a dozen as a minimum is about right, depending on how you care to carry them and how much personal mobility you figure is required by the probable situtaions you might face[ this being where I go for DPMS AP4/AR10 mags. And, of course, get all the ammo you can.
If you can't get your Vector V51, you might go with a PTR or Century CETME clone instead, of lower quality and corresponding lower price, but in the same general size as your AR10. But should it come to that, you can put on a H&K G3 collapsable stock or Choate folder, and surplus G3 magazines can still be had for 12 for $24 or so- 2 bucks each.
as for the Trijicon, yeah, they're good, and yes they are pricey, though not quite as bad as top=of-the-line Elcans or US Optics stuff. But if nothing else, pick up a $200 surplus British SUIT, which requires no batteries for operation, as an intirm placeholder or cheap insurance in the event of Trijicon damage.
Thanks for the advise. I really wanted the Vector V-51, but it was just too much trouble to obtain.