This is Drawing Extra Cards for player A, the player committed a Game Play Error by putting the Vampire Nighthawk in his hand instead of on top of his library. The mistake itself is the card going illegally to player A’s hand, there is no previous GPE or Player Communication Violation and the objects on the stack were resolved in the correct order.

We can downgrade in this situation the penalty from Game Loss to Warning because the MIPG has a provision that allows us to do it:

MIPG 3.4 – Philosophy: […]If the identity of the card was known to all players before being placed into the hand, and the card can be returned to the correct zone with minimal disruption, do so and downgrade the penalty to a Warning.

The identity of the card is known to all players (it was revealed by player A) , and we are at a point were the game state allows us to return the illegally drawn card to the correct zone with minimal disruption, so we do it and issue a Warning to player A. And don’t forget to issue a Warning for Failure to Maintain Game State to player B!

Please note we downgrade here because of that specific provision, not because the player called it on himself. Anyway it would be too late for that, the player called a judge after he could have gained an advantage.