It doesn't simply re-use SOME ideas from Van Buren. It uses almost ALL of them. The only things that have changed are the locations and most of the stuff from Mr House. The triad balance of power (BoS/Caesar/NCR) was reduced to just Caesar and NCR as well. Oh yeah and the Helios One facility I think came out of the idea to have the PC travel to a spaceborne weapons platform.
Not really. Only one location (Hoover Dam) was in both Van Buren and New Vegas. In Van Buren it was nothing like it is in New Vegas. The Legion wasn't even a major faction in Van Buren (they had already fought and lost the war with the NCR and most of the Legion had moved East). There was no Vegas in Van Buren. There's no Boulder Dome, no Nursery, no Tibbets, no Fort Abandon, no Denver, no Daughters of Hecate, no Mesa Verde, no Burham Springs, etc. in New Vegas, the main plots are totally different and even the background lore is completely different (the BOS-NCR war).
You can't compare gameplay and mechanics because games evolve. By that thread of logic neither FO3 or FO:NV are actually Fallout games at all. Its an old and pointless argument that fallout games are supposed to be from an isometric view, which svckED at the time and svckS now. Technology changes as time rolls by and the games we play change with it.
A lot more changed than the player's perspective. RPGs can "evolve" in ways other than adding in shooter elements, minigames and a sandbox world.
The reason people say that New Vegas is the same as Van Buren is that the themes and general ideas and most of the main elements remained unchanged. All that Van Buren was has been incorporated into New Vegas. Since NV isn't a sequel to FO3 they didn't take much of that game into NV. VB was going to be a direct sequel to both FO1 and FO2. NV is that sequel so in a VERY real way NV IS Van Buren after its evolution of technology.
They really didn't. A ton of stuff that was in Van Buren did not make it into New Vegas. As for the notion that they didn't take much of FO3 into New Vegas I think it's self evident how much that isn't true. The games are extremely similar.