vellum hair thickness

To randomise vellum hair thickness, you use pscale values. To get Vellum to recognise these values, UNTICK the Thickness box in the Vellum Constraints node.

polywire radius

To vary the thickness of your polywires, simply type in the @variable you've made into the "wire radius" field of  the polywire node.. A rather simple thing that I should have figured out *facepalm*.