As Vicious Rumors resolves, it deals 1 damage to each opponent, then the next opponent in turn order (or, if it’s an opponent’s turn, that opponent) chooses a card in hand without revealing it, then each other opponent in turn order does the same. Then the chosen cards are discarded at the same time. Next, each opponent puts the top card of their library into their graveyard at the same time. Finally, you gain 1 life.

In a Two-Headed Giant game, Vicious Rumors causes the opposing team to lose 2 life, each of those players discards a card and puts the top card of their library into their graveyard, and you gain 1 life.