Fallout Vault Boy is the mascot character of the Fallout series. He should never be confused with Pip-Boy who is the personal information processor in Fallout, Fallout 2, Fallout 3, Fallout: New Vegas, Fallout Tactics and Fallout 4.
Based on the developers of Fallout, Tim Cain and Leonard Boyarsky, the mascot was always called Fallout Boy or Vault Boy, but never Pip-Boy. They explain that the confusion must have come from the fact that the Fallout Tactics (Micro Forte) developers mixed Pip-Boy with Vault Boy and called him the former instead. The named ended up being used in the Fallout Bible by Chris Avellone.
The creators of Fallout 3 still continued to call him Vault Boy in the game, even though trademark legal documents referred to him as Pip-Boy.