Actually Vodafone's "netlock" doesn't distingusih by postpaid or prepaid, but by the IMSI range of your SIM. The IMSI of your SIM depends on when it was issued and may change if you swap SIM cards (e.g. due to a defect).
That makes it nearly impossible to predict if a Vodafone SIM will work in one of those netlocked prepaid phones or not. So the proof of the pudding is in the eating.

Aha. Are prepaid and postpaid (usually) assigned to different IMSI ranges, then?

Yes, at least that's my conclusion from many reports on telefon-treff.de.