I'll add a few more people who could make it to Vegas from the East.
Charon - he's certainly tough enough to survive the trip.
Harkins - again tough enough to survive the trip,... but he might be saved for a role in FO4 and the Commonwealth.
Fawks - can survive the trip and has a reason to go to Vegas, or Jacobstown more likely
Anyone else showing up in Vegas from the east would have an increditabily stupid reason for being there as far as I can see.
Harkins would be running security Rivet City no reason for him to go
Charon also has no reason, he could make a nice living in DC
Fawkes unless he tried following the sun one day and ended up walking mindlessly to Vegas. There's no way he would learn of Jacobstown in four years.
Most likely people to show up again are the Brotherhood, Lyons or Outcast
Maybe Enclave like Colonel Augustus Autumn coming back to the West to make a new life.
Or that Ghoul from Point Lookout, Desmond