Historical Rules · 5 October 2018
Rule 902.5a
Official source text from the 5 October 2018 Rules release
If a player takes a mulligan in a Vanguard game, just like in a normal game, that player shuffles their hand back into their library, then draws a new hand of one fewer cards than they had before. (In a multiplayer game, a player’s first mulligan is for the same number of cards as they had before.) See rule 103.4. Example: The hand modifier of a player’s vanguard card is +2. That player starts the game with a hand of 9 cards. If the player takes a mulligan, they draw a new hand of 8 cards. The next mulligan is for 7 cards, and so on.