Yagorath's Caustic Spray deals damage in an area, and will hit your ult drones if she shoots at you. Try to stay out of range or out of sight so it's harder for her to target you.
Vivian has some of the highest sustained DPS in the game, especially with her ult active. If an ally gets hit by Yag's ult, you'll easily be able to shred through her ultimate health to save them.
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.