From The Vault

Fallout Canon are the things considered to officially be part of the Fallout universe.

Fallout is the ultimate canon. Everything in this game is taken as the truth about the setting.

Fallout 2, whilst often viewed as an inferior title by the hardcore fans, is also considered canon if it does not contradict Fallout. If this happens, Fallout's version of events is taken as the truth.

Fallout Tactics is not considered to be Fallout canon, due to its many inconsistencies and contradictions. If something in Tactics does not contradict Fallout or Fallout 2's version of events, then it can be considered somewhat canon.

Some of the background information in Fallout Bibles is considered canon, except for some pure speculations by Chris Avellone, of which some might be canon or semi-canon, and some are clearly wrong.

Fallout: Brotherhood of Steel is so far away from canon that canon has forgotten the game exists, and if it remembered, would only send feces in the mail to the game's developers.

Van Buren was never released and the little we know from the developers is not considered canon, merely an interesting aside.