I shall answer by giving the easiest example:

For sulfur-crosslinked natural rubber (N.R), my students used to determine the crosslinking density  or the percentage of sulfur incorporated by dividing the number of moles of sulfur used initially on the number of moles of N.R which in turn is found by dividing the mass of N.R on the mass of the CRU "constitutional repeating unit" of N.R which is isoprene , as you know. Note that all sulfur is used up as if it is a limiting reactant.

I must add that the famous Ebonite "which is black -usually- polished hard rubber"  is N.R that has been vulcanized by using 30-35% sulfur for preferably one hour or more .

Thank you Prof.

how to calculate VR-volume fraction of the vulcanizate in the swollen sample.

I measured the molecular mass between cross-links Mc of epoxy via DMA here: 10.1002/polb.24077

However, the standard formula of the rubber-elasticity theory should work much better for rubber: 3E = rho R T / Mc

Here, E is the Young's modulus at Temperature T, R is the universal gas constant, and rho is the rubber's density.

Thank you Dietmar Haba.