I purchased the Varmilo VA21M from Caps Unlocked, and out of the box, it would not work properly with either my iMac or Macbook Pro running macOS X Sierra (10.12). However, I found a 3rd party driver that makes it compatible. I downloaded & installed USB Overdrive
http://www.usboverdrive.com/USBOverdrive/Information.html, a preference pane for macOS compatible with Sierra and previous versions. Once I plugged in the numpad and added each key...it worked. The driver would presumably work for full-size keyboards, too.
It's worth noting that neither Karabiner nor Karabiner Elements worked. In fact, I use Karabiner Elements on my iMac for my dasKeybaord 4, and I believe that it conflicted with USB Overdrive, preventing the VA21M from working on my iMac. On my Macbook Pro, however, I do not have Karabiner Elements installed, and the VA21M works properly on it through USB Overdrive.
USB Overdrive is distributed as shareware, but it's easily worth the $20.00 license fee to support its development and keep it updated for future macOS versions.
Also, I'd like to compliment Caps Unlocked's customer service for their willingness to work with me to find a solution, even if it meant returning the custom-built numpad for a refund. 10/10 would buy from them again.
Disclaimer: I am not affiliated with any of these products except as a satisfied customer. I'm posting this in case it helps other people find a solution faster than I did.