I recently set an association between a Zen21 On/Off switch and a GE On/Off Plug-In Smart Switch. The Zen21 (trigger device) is used as a wall switch that controls a lamp pluged into the GE outlet.
Zooz provided directions as follows:
To set direct association, click on the trigger device in the Vera dashboard, then go to Device Options and under Associations enter 2 for Group ID, then Add Group.
Then click on Set and select the device you’d like to trigger.
If you’re dealing with battery devices in this set-up, you need to wake them up for the settings to go through.
These instructions are slightly different than what is found on MiOS Wiki. (though that page is from 2016)