All creatures your opponents control are affected by Vedalken Humiliator's ability, not just those controlled by the player it's attacking.

Vedalken Humiliator's ability overwrites all previous effects that set an affected creature's base power and toughness to specific values. Any power- or toughness-setting effects that start to apply to a creature after the ability resolves will overwrite this effect.

Any power- or toughness-modifying effects (those that give +N/+N, for example) and counters will apply to the creature's new base power and toughness, even if they started to apply before Vedalken Humiliator's ability resolved.

If you don't control three artifacts as Vedalken Humiliator's ability resolves, it has no effect. If you do, the effect continues to apply even if you lose control of Vedalken Humiliator or your artifacts later in the turn.

Vedalken Humiliator's ability affects only creatures your opponents control at the time it resolves. Creatures they begin to control later in the turn won't lose power, toughness, or abilities.