Hi Jason,
If you take a look at this other thread, you'll see that @dms.md.us reported similar behavior when connecting the VRCZ4 to other z-wave dimmers -- and that while I hadn't noticed it initially, the same thing seems to be happening for me (on the only z-wave dimmer I connect it to) since I migrated my setup from Smartthings to HE.
I'll be looking into whether I can restore proper z-wave dimming shortly (within a few days, I hope?), but in the meantime...what are you controlling? Are they z-wave or zigbee devices?
Almost all of the switches on my 4 VRCZ4s are controlling Philips Hue bulbs (or groups), and they're all dimming perfectly (which is why I hadn't noticed the new issue with the z-wave dimmer). I have a couple of thoughts on what might be happening with the z-wave dimmers, and will let you know what I find.
Oh -- and I'd never heard of the IWC600 before. Looks interesting, but I wouldn't expect the VRCZ4 driver/app will be much use there, as it relies at least in part on proprietary Leviton commands. That said, I'll take a further look at the IWC600 at some point and see if it looks promising...
