New Vegas doesn't exist without Fallout 3.
And Fallout 3 doesn't exist without...
People need to stop using the "Obsidian only built upon Bethesda foundations" line. It's bullshit. Just stop.
I can't believe people are seriously trying to argue this. New Vegas was absolutely built off of Fallout 3. They made an expansion pack the size of a full game, they didn't make the entire game from the ground up themselves.
Obsidian took Bethesda's clay and sculpted it into something amazing, something a lot of people prefer over Bethesda's sculpture they made with their own clay. But here's the thing: Obsidian doesn't know how to make the clay, they only know how to sculpt it really well. And that's ok.
More than ok even, because it means we could have another situation where we get a Bethesda made Fallout for the people that liked Falllout 3 better, and then an Obsidian Fallout for the New Vegas fans.
We can all co-exist here, it doesn't always have to be about which 3D Fallout is better. Fallout 3 and New Vegas play to different strengths, and they focus on different things. Some people put more importance on the things Fo3 does well, and some put more importance on what New Vegas does well.