A zwave or zigbee device can only belong to one primary controller.
It’s technically possible to set up SmartThings as a secondary controller to Vera (much like the Aeon Minimote becomes a secondary to SmartThings), but the results are rarely quite what people want, so it isn’t done often.
For example, many devices rely on Association to report status, but can only be associated in one group. So even if Vera and ST agree between themselves that one is primary and one secondary, only one would get the status updates from say, an eco-link sensor.
Also, locks are a separate category because of the security requirements and this also tends to mean they will only report to the primary. You can sometimes get around that with virtual switches as status indicators but it gets really complicated.
Before we get too far down the path of what you might do with multiple controllers, there are community developed smart apps and device handlers that provide a lot of custom code management for locks. They are certainly worth looking at.
But they still don’t give you all of the functionality the lock is designed for, so it comes down to exactly what you need.
Lock Code Manager is very popular, for example: