Flights from Vancouver are definitely much easier to find J award seats on than LAX or SFO, and involve a much more user-friendly routing than going via Asia (unless Asia itself is of interest), plus some FFPs won't allow you to route a North America to Australia/NZ award via Asia. Having said that, flights from PEK, PVG, NRT and KIX have much better premium award availability than any of the North American routes (though beware that sometimes KIX and NRT get the 763 which has a vastly inferior product).

As others have said, often the best availability on LAX/SFO is relatively last minute. Either way, if you have expertflyer you should set up alerts for flights that are of interest.

Whatever routing you book, you should start paying close attention to availability from about 2 months out - NZ often releases more inventory (esp in premium cabins) 2-3 months out, and then again closer to the time if the flights are at all quiet. That can be a great way to grab a better routing if you are a little bit flexible on timing, though often finding premium inventory within the US (e.g. on US or CO transcons at compatible times for the NZ international flight) is very difficult by then.

Finally, another option if you don't mind the older style product is to connect via one of the Pacific Islands. RAR makes for a very pleasant stopover.