The Welcome to Fabulous Las Vegas sign was created in 1959, which is almost right before the divergence. It's also shown to be modified to replace "Las" with "New" in the marketing ad with the NCR Ranger in the front. But the question is, since the sign supposed to be at the south entrance of the strip, has it been replaced with this?
http://cache.gawkerassets.com/assets/images/9/2010/08/500x_nv4.jpg
The real world sign hasn't been shown in any in-game screenshots other than one ad so is it possible that it won't appear in-game? I've always thought of the real world sign as a symbol of Las Vegas anytime you saw it on TV or something. Does New Vegas need the real world sign?
EDIT: I can only find an example of the modified real world sign ad in a gameinformer mag right now so I can't link it.