Not sure if many people know this thought i should share it anyway.
1 Make a portal before you enter the last staircase to Vaal.
2 Go to Vaal and wait for your party to activate the boss.
3 Leave through one of the party members portals.
4 Enter your portal and wait for Vaal to die your quest objectives will alert you of this.
5 Go back to town and back to Vaal via your parties portal.
You can go down the stairs and do this aslong as your party has not activated the boss already.