It's not clear to me who is operating the SGN-SYD flight : VN or CX ?
For the regional flight, as you said, you have to credit to VN as you won't earn anything with DL on a CX-operated flight. For the long-haul, assuming it is operated by VN, is it on the same PNR as the regional flight ? If yes, I don't think you can credit to 2 skyteam programs depending on the flight (VN first then DL). Stick to VN all the way.