In order to apply values to your mesh VCC has to find out with parts of the mesh you're attempting to work with, this involves checking every vertex in your selected meshes to find out which vertices require new values. This is very quick on a collection of low poly meshes but once in the tens of thousands of verts expect a few seconds and so on (depending on your system specifications of course).

• Check the add-on preferences setting called “Preset Directory” is set to the correct directory and that the saved JSON file at that location has not had it’s name changed from “VCC Presets.json”.
• Close and re-open your current blend file, presets are checked for and loaded when a blend file is opened.
• If both sets of presets are empty and a new preset is added, this will also cause the add-on to check for pre-existing presets.

• If you’re using the VCC Channel Viewer material make sure you’re using the correct channel, this can be checked at the bottom of the VCC panel.
• With the mesh selected, go to the “Object Data Properties”, look in the “Vertex Colors” tab, if there is more than one layer, check that the correct layer has the camera icon to it’s right highlighted, this makes it so that layer is the “Active Render” layer.

• Try deselecting your mesh and reselecting it, then apply the values again.
• See the entry for “Can’t see the vertex colours you’ve applied?”.