Rule 902.5a

If a player takes a mulligan in a Vanguard game, just like in a normal game, that player shuffles his or her hand back into his or her library, then draws a new hand of one fewer cards than he or she had before. (In a multiplayer game, a player’s first mulligan is for the same number of cards as he or she 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, he or she draws a new hand of 8 cards. The next mulligan is for 7 cards, and so on.

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