The issue on continuity and gameplay was not discussed at all here tho. By all rights fallout 3 is *much* harder than fallout new vegas. Somewhere in the middle of the story you get to fight enemies fully clad in energy weapons (which with new vegas now ignore portion of your DT) and power armor (the highest DT armor in the game), before that you get to fight super mutants, which by all rights are at least twice as tough as your average raider out there.
Compared to new vegas, which is really easy throughout the main quest, and arguably harder on the DLC's (which i'd say are about the level of the DC main area).
From a gameplay perspective, it'd be much easier to start in vegas and move on to DC, as to not have a weird difficulty curve (steep at first, all the way down again and then back to difficult).
I like the flashback idea, but i'd imagine it'd be hard to explain how your gear can "flash back" with you. If you say that the wanderer heads out of his vault, and straight to the train, james would probably be okay for a couple of years (he is in that simulator after all), so i'd most likely go with that. However, some reason as to why he'd be motivated to leave his father in the wasteland and run for vegas, is something i have a difficult time coming up with.
Right now, i usually go to new vegas right after james dies. It does not make sense from a story perspective (unless you'd argue that the enclave and BOS fight it out for a couple of years in a standstill), but my character has a proper motivation to tell the DC wasteland to fuck off, and right about that time i am incredibly resource starved, usually on my last 5 stimpacks. I launch an attack on the train station in a last desperate effort, and just roleplay that my character got amnesia, not from the bullet from benny, but somewhen before. (after the first time in the divide, makes sense).
Wacky? Yes, most certainly it is. It makes no sense, story wise. Gameplay wise, if i'd boss through DC and then went to new vegas, i'd be bored out of my skull there. I'd probably have fun in the DLC areas again, but i'd like everything to be fun if possible.