Vivian’s only way to do damage to you is her gun. When the ultimate is up, she’s really dangerous.
Her sentinels have AoE, and their size is kinda big, too.
Try to focus her sentinels when her ultimate is up, so she isn’t so bursty.
If Vivian uses her shield, aim for her feet if you’re running direct damage, or the ground below her if you have AoE. This is extremely useful in any range combat with her.
Vivian's shield only protects in front of her, use a flank champion such as Skye to attack her from behind. With any champion you should still try to attack her if doesn't notice you.