First of all, I'd just like to say that I absolutely love the mod.
Second, I'm not sure if it'd be possible, but I had an idea to better integrate the travel between DC and Vegas. I played a lot of the Elder Scrolls games, and they typically use letters for introducing new areas, questlines, important NPC's, that kind of thing. It gave me an idea for this mod.
After some reasonably early quest (I was thinking saving or destroying Megaton) you could get a letter from "An Anonymous Benefactor" asking you to come out to Vegas to make a delivery for him. Included would be a train ticket, and it would start a quest that would help you navigate to the train station.
Starting in the Mojave would be a little harder to figure out because it's kind of time travelly, but that way makes sense at least. (Maybe you could get a letter after dealing with the Powder Gangers in Goodsprings one way or another, saying that your dad had heard about your exploits and would love to meet you)