Hey Jeremy,
I know this topic is a little old but I’m doing almost the exact same thing as you (minus the Nest). Vivint is my secondary controller while SmartThings is my primary. It would not work for me for me when I first tried it so I forgot about it and let it sit for 6 months. After several firmware upgrades though, it seems to be working now!
I just commented about it on another post here: https://community.smartthings.com/t/vivint-smart-things/75781/5