warning potential tl;dr ahead
I have only done rudimentary mods for my own use and am in no way a designer, but I am good an analyzing situations and making narratives and connections.
From my stand point there are a few questions that need to be asked.
First, in terms of sequence, NV takes place long after Fo3 so any arguments in terms of continuity or lore need to be placed on the side if it is decided that we want individuals to be able to move freely from one zone to the other.
If we forgo this, time become an issue. With Dead money, how many coins will an individual be earning simply by moving back and forth? If its anything but high speed travel it could be potentially a cheat system.
further if individuals can move freely, what is happening in each area over the vast swaths of time the courier/lone wander is moving about.
Given these time components, I find it hard to agree with anything except a teleport. however this necessitates both a DC start and having completed mothership zeta.
I see a few viable options, the courier is a descendent of the lone wanderer, and in a very Assassins creed fashion, DC takes place in dreams.
This would solve temporal issues, but raises issues of equipment.
Thus, dreams are not good.
The teleport has issues.
The vertibird has the same problem as the zeta option.
The train takes to long. And we have exhausted ideas.
Thus I have a recommendation.
A Dc start with a quest to become a courier. This quest can be engaged any time between leaving the vault and the first encounter with your dad and after the retaking of project purity. This solve means and time. The most that would be added would be a DC express branch that offers a high paying and tantalizing job. Bring this chip out west and link up with the Mojave express. Once accepted New Vegas can begin. Now as for the return trip. We can implement the train. It will just require a bit of addition. For those of you that know what COG is skip below. COG stands for Continuity of Government and is a current government program by which parts of the US are designated as alternative operation centers for peace time facilities. One of these Facilities is Raven rock :) yes folks Raven rock is the alternative site for the department of defense in the event of full-scale war. The one closes to the Mojave is Cheyenne Mountain in Colorado.
Now with that explained, I see two options. First we can incorporate the dog city idea, and use the explanation of COG to link the pentagon and/or Raven Rock to Colorado and then through a train or Dog city to the Mojave. Or using the hand wave of story telling, explain that it was also connected to somewhere in the Mojave area.
What was doing the connection? An underground railroad. Though currently in the realm of conspiracy theory, the idea of an underground transcontinental railway could easily be passed as lore friendly, given that that the system was designed to withstand a nuclear war, we can say that the facilities are still intact and the only remaining problem is time and propulsion.
Mode of movement can be hand waved as well, (underground nuclear power plant) (cold fusion) (fuel depots) (geothermal) (tesla tech) sky is the limit really. In terms of time, if its underground there will be no obstacles and three day transcontinental freight is far from unheard of. And this would also solve our time problem.
Lastly cog was implemented in the early 50’s and the latest day of timeline divergence is early 60’s so in terms of lore this also is functional.
Sorry for the Tl;dr, but hopefully it will spark ideas.
edit: also, if need be, the common wealth can be used for hand waving and/or a source of deus ex machina.