virtual master is super cool, but for a simple variable font(masters are in only in both side of axes), when number of axes goes high, number of side per axes masters(1,2,4,8,…)+ zero master(0) grows very slower than all required masters, so required virtual masters grows very faster.
required side per axis masters + zero master = axes number+1
all required side axis masters = 2^number of axes
for a 5 axes variable font we need 5+1=6 masters, and 2^5=32 is all required masters, so required virtual masters are: 32-6=26 masters! setting all these virtual masters via cp is a little hard(and getting harder with higher axes numbers).
All axis defile a space. Glyphs needs to know how far each axis goes. In most cases that is define by the normal masters. Only if you have axis where you only have brace layers, you need to add virtual masters. And you can use one virtual master for several axis. Use one for all the upper and one for all lower bounds.
Virtual masters are useful in case where you have an axis like “Ascender Height”. Because you only need extra drawings for a handful of glyphs, you don’t like to add a full masters but you add some brace (intermediate) layers. And then you add a virtual masters to tell Glyphs the range of that axis.