If they codeshare, you'll be able to travel on the same aircraft with either a V Australia or Northwest ticket.
Hopefully the codeshare goes both ways, so you can book with V and travel on NW flights in Americaland, in which case it will mean that travelling domestic first and then international, you'll be able to check your baggage through.
There's never going to be automatic check-through of baggage from international to domestic, because you will need to physically present your bags at customs upon the arrival of an international flight. This has nothing to do with codesharing, and everything to do with customs and immigration.