My understanding is that IATA sets rules that state that for checked baggage involving multiple carriers the rules of the most significant carrier apply for the remainder of the itinerary (which in this case would be the segment from LA to Sydney since it crosses IATA zones). Indeed the Consumer Transportation Agency of Canada confirms this is the case, and as the regulator of air transport in Canada would have final say:
Issue The Agency needs to establish a clear and transparent approach to the application of baggage rules (including checked and carry-on bags) in travel arrangements involving multiple air carriers for transportation to, from and within Canada, in order to reduce confusion for consumers and...
otc-cta.gc.ca
Remember you aren't buying two tickets Vancouver to LA and LA to Sydney. It's one ticket that just so happens to connect through LA. Keep in mind too that departing Canada by definition all rules and regulations of IATA and ICAO have to be followed by them. Why? Because both organizations are based in Montreal. It would be very foolish to host an agency like them yet not follow their rules. Indeed, some airports in Canada are implementing the bleeding edge IATA rules (such as the One Stop Security IATA annex that make international to domestic transfer dead simple).
-RooFlyer88