To randomise the stretch rest length value on initialisation (not dynamic, it can't be animated like this). make a primitive wrangle between the vellum hair constraint and the solver.
I've used a connectivity node set to Primitives and changed the "class" attrib to "hairID". Here we affect the restlength value by multiplying it by some remapped randomness.
f@restlength*=fit01(chramp("remap",rand(@hairID)),chf("min"),chf("max"));
