Assuming you don't have more important targets, hold your ult for when Vivian uses hers. Tag her with a bit of damage, then ult. Be wary of supports being nearby, or if she's running the Scapegoat card, as either will likely shut down the kill
I find blinking out of iceblock to have more of a delay than soaring out of iceblock, but soaring is easier to shoot. It really depends on where you are, who is looking to end you. Not staying for the full 3seconds CAN mislead the enemy too.