Van Buren is the codename Black Isle Studios assigned to their second iteration of Fallout 3. It saw development by Black Isle Studios until its cancelation in December 8, 2003 and would have been published by Interplay Entertainment.
It takes place in the United States of America during the Great War. The player character in the demo is referred to only as Citizen, though they share a model with the Prisoner. The Citizen made their way to the relocation center and was escorted to Vault 1 by Corporal Armstrong of the 4th or 13th Armored Infantry Division. To reach the vault, they must fight communist insurgents. After they enter the vault, they must help turn on the life support system.
The URL I linked with this post is to the ModDB page. It’s an interesting novelty.
A lot of elements from Van Buren would go on to appear in Fallout New Vegas.
Multiple mods have attempted to recreate what could have been using the leaked design documents and interviews for inspiration.
I think Fallout Yesterday might rival it. They mentioned getting some additional information regarding the plot from Chris Avellone in the NMA forum post and they released an update this month as well.