While it was mostly (or by some accounts entirely) Bethesda's fault, Obsidian never really did efficiently redesign the game enough to make it completely improved from Fallout 3. Granted, the base engine and design for Fallout 3 was screwed from the very start so maybe that was an impossible task, but if we're talking technicality here, Fallout 4 looks and feels far more like a sequel than New Vegas is.
Why is everyone so insistent that New Vegas being considered an expansion is detrimental to it? As a sequel, a game with slightly reworked progression, a couple of new crafting-related mechanics and one new mode doesn't make a very good one, story ignored in this discussion. As an expansion, it adds more content, more areas of gameplay, and is overall more tight and balanced in terms of exploration and combat. That makes New Vegas an excellent expansion to Fallout 3 but an ineffective sequel to Fallout 3. I feel it's more of a compliment to think of it as an expansion.
I'm not going to take the writing into consideration since "more plot, better plot" isn't exactly the only requirement for a good sequel, and for the sake of the comparison I'm ignoring the fact that Fallout 1, 2, Tactics and BoS ever existed, as they sit in completely different genres. All in all, New Vegas makes a very good standalone expansion to Fallout 3, and that's where my opinion stays. How New Vegas stands as a game on its own, as a Fallout game, as an RPG, etc., wasn't in this argument.