No, New Vegas isn't a 100% new game, I agree. The game itself (As in what you interact with, what you see etc) uses a lot of Bethesda's assets, but there are just as many (If not more) items created by Obsidian, from character facegens to faction armour, from weapons to buildings. The time they saved on creating new items (Most of the reused ones are smaller items such as pilot lights, stimpaks, etc. which could have taken a lot of time to make) could have been used to create the needed new items and on crafting the game. New Vegas had a smaller dev cycle than FO3.
It's no less of a full game, though, and to call it "a conversion of an old game" isn't accurate, or fair, to any degree.
I'll also agree that Obsidian should have cut down the resources a lot, because if you're right (And I assume you are from Vometia's comment) then there's a lot of fluff that needs removing to streamline the system.