Van Buren is owned by Bethesda. They even put some references to it in FO3, including the Chinese stealth armor.
I'm also not certainly what would be left if Bethesda could use the Van Buren background, fluff, and world but not the "work." I suppose the actual game code but then it's not like Bethesda has plans to use that anyway (even though they could if they wanted to).
As for this potential leak, the format is very similar to the early Fallout 3 previews . . . although I suppose that could just as well be a strike against its authenticity as for it.

I'd love to play as a ghoul so I'm certainly hoping this is real (or if not that's in the game anyways). I don't really care about vehicles but if the game does have expansive (and largely empty) areas to travel through I'm sure I'll appreciate them.