Yes - automatic fallback to GSM networks will occur with all the 3G handsets. This has to happen, otherwise 3G users on 021 would only have coverage in a few places (Vodas 3G areas only).
Dont buy a trademe or parallel imported handset without reading about the problems first. You might pay 10 - 40% more to buy one in a shop, but there are some cheap 3G handsets now ($440 for a new video calling handset I do beleive), and local service is (nearly) always worth the extra.
Vodafone wont provide support for imported handsets (and rightly so).