In a Two-Headed Giant game, your life total is the same as your team's life total, so Eternity Vessel enters with a number of charge counters on it equal to your team's life total. When its landfall ability resolves, it causes its controller to lose or gain the appropriate amount of life, and the team's life total is adjusted by the amount of life the player gains or loses as a result of this ability. Suppose Eternity Vessel has 15 charge counters on it when its landfall ability resolves, and its controller's team has 11 life. That player's life total is 11, which becomes 15, for a net gain of 4 life. The team's life total becomes 15.