If you are layering clothes, say a jacket over a shirt, is it simply a matter of making the Mass property of the ‘undergarment’ larger before you do the Vellum solve? or is there more to consider?

art3mis
or is there more to consider?

you can consider using the actual layer attribute
just turn on Layer parameter on Vellum Constraint and set it to whatever you need

you can also adjust Layer Shock parameter on the solver

both parameters have tooltips with more info