You are claiming that a road sign with Washington D.C. would be all the way 3000 miles away from D.C. Vegas would point the same way as D.C. too if they just so happened to be together, which they aren't.
It would more be a symbolic sign showing the 3 games they would have made/be making. Fallout 3 in DC, New Vegas and then possibly San Fran. No need to have the arrows point in the right direction as the sign would be symbolic... Plus I'm sure someone as inclined as myself and whos familiar with California could find you a point in the state where said sign would be correct (Nothern california would display with 3 seperate directions as vegas would be S/SE, DC would be E and San Fran could be N/NE)
Fallout 2 has already visited San Francisco, and the NCR basically calmed everything down. They would change the art as it has been 3 years since Fallout 3 released, 4-5 when Fallout 4 will hopefully come out, if they keep the same grpahics, it will look terrible with everything else.
Your confusing Black Isle's Fallout 2 to everything gamesas has done in the FO Universe. They only honor the past/cannon where they see fit and how they see fit. From their standpoint, San Fransisco is an untouched city for them to base a game around. Personally I would LOVE to go to San Fran because then the Brotherhood of Steel would be a major faction to side with instead of just being relegated to side quests that connect a storyline quest and/or a way to get power armor training. It would make sense for them to go from Vegas to an NCR Area.
As for your graphics thoughts, Others have pointed out the difference between Art Styles and Graphics. A Highwayman will NOT instantly look different because they used a different engine. They have to keep their version of the Lore going by making things similar to their original counterparts.
INot only that, but the NCR would have cleaned it up better then that. Heck even if the Legion wins the dam there is no way it would strike that hard into the heart of the NCR.
If you listen to Cheif Hanlon and several other NCR officers, You come to realize that the NCR has most of its assets dedicated to holding the Dam. Should the NCR fail at the Dam there would not be much standing between Shady Sands and the Legion. You'd have small pockets of resistance but the Legion and its blitz style of warfare would make it deep into the NCR before the slow moving NCR senate could formulate a plan or place a new general in charge to defend. So yeah, Its possible for the legion to make it far far into NCR territory in a quick fashion if they go with the story line of the Legion actually over-running the Dam or that the Legion after its defeat went guerrilla on the NCR.
The next Fallout is Fallout: Online and is has been announced to come out in 2012.
I actually hope FO: Online comes into being but with the current trend, gamesas will keep attempting to stop if from ever coming out which if they do invalidates their ability to make any games in the FO universe after FO4 (Unless they some how find a way to negate the contract they signed when getting the rights to Fallout... it would result in a rather long lawsuit during which time both sides would more than likely file for injunctions to stop any Fallout games from being published)