To be fair we don't actually know what Obsidian wanted to make with Fallout: New Vegas. Would they have preferred an isometric game? Maybe. Did they want to make a Bethesda-like game? This is also possible. If it's one thing I'll say about Obsidian it is that they aren't afraid of trying new things as risky as it is for them.
Thats a good thing then, and your right we have no idea what scope or scale they Really wanted to make. Given the history with Van Buren, I would imagine that they have been thinking and dreaming of this for a long-time now. It would be hard to Not take the scale too-large, and with the time they have their options are limited with this release.
What I also put my hopes on is the fact that Bethesda has stated a Dozen times at least that they are by no means done with Fallout. This could mean Fo4 from Bethesda, but it could also mean FoX from Obsidian again - assuming they do a good job on New Vegas and that it sells enough-copies to satisfy the bean-counters.