Mask Image by Brightness is a Luma Key (which, in Red/Green/Blue mode, evaluates those color channels individually) and not a Chroma Key (which evaluates distance from a specified color); it's not great for Chroma Keying. Our community members have had a previous discussion on Chroma Keying.
To elaborate, Mask Image by Brightness measures the brightness based on the Threshold Type you select — if you select Blue, it only measures the Blue channel; parts of the image that have no blue (such as the green background) are made transparent, and parts of the image that do have blue (such as the dog's gray collar) are left opaque. If you select Green, then parts that have green are left opaque.
The plug-in works fine for us. We wonder if it's some type of interference with some of your other 3rd-party plugins. Could you temporarily remove them, and see if the problem resolves? If it is interference, and you can narrow down what plug-in(s) are causing it, we can hopefully figure out why that's happening and fix it.
Since the MIDI mappings are implemented on the VDMX side rather than the Vuo side, we think the fix would have to be implemented by the VDMX developers. We've sent them a link to your bug report. We'll let you know if we hear back from them.
uploaded .zip contains Frosted.fxplug, and Motion Template (following Finder link from export success window) - although i notice isn't making a folder with the moef, but a Package instead. not sure if that matters.