Hard rule specifically said by the original devs, or something you think since it applies to many games?
Ok then lets say Easter Eggs are canon. Ok that means Godzilla, Dr.Who, Star Trek and the Chosen One going back in time are all canon. You can't say "well the alien encounters are canon but the Star Trek one isn't" They're all canon or they are all none-canon.
Edit: Since the Alien encouter in the orginals took place in a game with Star Trek encounters and Godzilla encounters it's safe to say the devs didn't entend for them to be canon.
Same with New Vegas. First you need to have the trait to even encounter them (making it clear it's an easter eggs) and that alien craft is also with a "nuke the fridge" refrence.
"Wild Wasteland is a http://fallout.wikia.com/wiki/Fallout:_New_Vegas_traits in
http://fallout.wikia.com/wiki/Fallout:_New_Vegas. It adds (and in some cases changes) random encounters and locations with humor similar to that of
http://fallout.wikia.com/wiki/Fallout,
http://fallout.wikia.com/wiki/Fallout_2 and
http://fallout.wikia.com/wiki/Fallout_Tactics http://fallout.wikia.com/wiki/Special_encounter. Players without Wild Wasteland will find fewer humorous anologues in places of the Wild Wasteland encounters.
Events that are modified/triggered by Wild Wasteland are indicated by an "alien-esque" music and an on-screen message saying "..." with an image of the http://fallout.wikia.com/wiki/Vault_Boy with spirals for eyes, identical to the Wild Wasteland image."
It's clear that it's none canon for fun content.