As I understand it, Van Buren was designed over a much wider area, spanning Utah and Colorado.
Nearly the whole of Utah, Colorado, New Mexico and Arizona, in the style of Fallout 1 and 2, with travel through world map between locations. As the name suggests, Fallout: New Vegas will focus on one city and its surroundings, like Fallout 3.
Not to mention the fact that if it were a Van Buren remake, it wouldn't be called "New Vegas", because Van Buren didn't even include Vegas (although Hoover Dam is pretty close to it).