Er, no, not impossible. At least, no impossible to make a mod that uses the Van Buren story as a basis.
Sure, if you wanted to actually created an isometric-perspective, turn-based game... that would be ludicrously difficult, perhaps impossible, and pointless, would make more sense to use Fallout 2.
But, a mod that used the Van Buren story but was still a first person shooter would be quite doable.
People are already making some quite impressive stuff.
Look at Black Mesa (aka Black Mesa Source), a mod for Half-Life 2 totally recreating Half-life 1. That's a hell of a lot of maps, weapons, NPCs and dialogue recreated.
Of course, it has taken them about five years, longer than some retail games. Supposed to be released some time 2009.
Personally, I was never that impressed by the Van Buren concept and storyline, so I don't go around thinking "Oh, I so wish Black Isle had made Fallout 3." I think many aspects of Bethesda's Fallout 3 were a good idea, but needed better implimentation.
And I'd rather see a massive total conversion mod that recreated Fallout 1, rather than trying to make Van Buren. Fallout 1 remains my favourite in the series.