Once Siege Striker's ability has resolved, it keeps the power-and-toughness bonus even if some of the tapped creatures leave the battlefield or become untapped.

If Siege Striker gains vigilance and attacks or attacks and becomes untapped before its last ability resolves, it can be tapped for its own ability.

You choose how many and which creatures to tap while Siege Striker's ability is resolving. No player may take other actions between the time you make this choice and the time Siege Striker's power and toughness are raised.

If an attacking creature with vigilance is tapped for Siege Striker's ability, that creature remains an attacking creature.