Each opponent discards two cards. If you control a Vampire, each opponent loses 2 life and you gain 2 life.
Whether you control a Vampire is checked as Arterial Flow resolves.
In a Two-Headed Giant game, Arterial Flow causes the opposing team to lose 4 life, and each player on that team discards two cards. You gain 2 life.
Arterial Flow causes each opponent to lose 2 life if you control a Vampire, even if some or all of those players were unable to discard any cards.