A |S21| measurement is reasonable. However, the signature for a lossy choke should also be apparent in a S11 measurement. So take a short length of #22 AWG enamel wire and tie it across the CH0 port of the VNA. The inductance measured if it is reasonable length of wire, say 100 nH should follow a CW trajectory on the chart, along the 50 ohm circle (blue). When the lossy ferrite material is added, I would expect an increase in L as well increase in series R. The result is an increase CW rotation over the same frequency swept range as well a shift of the circle contour to the right. See figure (red) . Try it, see what you get.
Alan
