Well I tried simple scene without any, heavy thick glass caustic causing stuff. If I understand this right, this is not the kind of scene in which using VCM pays off, but wanted to try it just out of curiosity...
1: default VCM settings
2: default VCM without Merge function
3: large 24 pixel, initial radius default
4: large 24 pixel, initial radius without using MIS
The scene has in openings single sided object using 2sided material for glass.