The Fallout 4 trailer has arrived, and the first thing I did (of course) was immediately spam Google with whatever terms I could think of to match key scenes with existing Fallout canon. I had to know everything I could. Around the 90-second mark, there's a large metal door numbered 111, strongly hinting that this game will center around Vault 111 (all Fallout games relate in one way or another to various shelters where humans took refuge pre-apocalypse).

I did not find concrete information about Vault 111 or its whereabouts (presumably it's somewhere near Boston). I did, however, stumble upon Fallout fan fiction written in December 2011 about vault-dwelling kids discovering the truth about Santa Claus. That's when things get very, very dark. Here's just a snippet:

Skeletons of the deceased can be found all across the vault, even the overseer in a santa suit is found in the overseer's office. The song "Jingle Bells Rock" playes [sic] over and over in the dark depressed vault, when the song comes close to an end it begins to scratch and repeats itself over and over again.I don't think this is going to be canon.