Doesn’t look like anyone has had any luck getting the VRCZ4 (either generation) to work with SmartThings.
As far as having ST work as a secondary controller, even if you could it wouldn’t be able to issue the Leviton commands it doesn`t support, so I’m not sure what could it would do you.
If you already have a Leviton set up that you like but there’s something more you get from ST that you want, like control of zigbee devices, I’d consider just running two separate networks in your house.
I run ST for some convenience use cases and to get voice control (through Siri and IFTTT). I run a completely separate network for security. But different things work for different people.