Through out the Fallout Universe Aliens and there equipment have been with us the whole time. I believe that plasma weapons are reversed engineered alien blasters. They have been in Fallout 1, 2 and 3.
Also Area 51 is cannon on Fallout 1 so I hope it's in FO:NV
http://fallout.wikia.com/wiki/Alien_Ship
http://fallout.wikia.com/wiki/Alien
Those are easter eggs/special encounter"Numerous easter eggs and references to various books, movies and other games are one of the staples of the Fallout series. These are instances where something or someone from pop culture or the real world is referenced in the game itself, e.g. Fallout 2 has an encounter with King Arthur." wiki.
Special encounters are random encounters, often non-canonical, and are unique compared to more common encounters which may consist of hostile NPCs. They usually serve as entertaining points of humor, though some are very dangerous (Mad Brahmins), while some grant access to unique, sometimes powerful loot (Bridge Keeper). Most of these encounters consist of humorous pop culture references or other games or even earlier Fallout games (The Cafe of Broken Dreams).
Aliens are meant to be joke. The case for having them in fallout 3 mothership zeta because "they were in fallout 1/2" is false.
There is not evidence that the plasma weapons have anything to do with the alien blaster.
Alien does not have to mean little people from space or the aliens in the alien movies.
I can mean "strange unknown, not or the normal ie: Alien"
Alien blaster could be just that!
Even the one you find in the easter egg. It's just an easter egg.
If we include aliens form the special encounter we have to include Star Trek, Dr. Who and godzilla!
Having Aliens star the great war are is very bad for fallout. Fallout 2 intro makes it clear people and only people cause it.