I know that you can change the booking as to where to earn points for the whole trip, I have an AirNZ booking coming up, and as mentioned above, have decided to put all my flying to KF, as we wait and see what pans out after Oct, and in the booking, when I go into it, with the 6 alpha numerics with the last alphabet ending with H, it says on a 2nd page, where the points for the earn goes to.
I guess in the end, you could change the booking in the AKL Koru if you have access to that there.
Not sure if the booking as to where points accrual goes can be changed on a mobile device, but on a laptop, it shows.
I would say, its a give and take, if you do half and half, you loose the points going to KF, but you earn VFF points and VFF SC on the MEL - AKL sector.
If you just put all the points to KF, and ignore the VFF SC earn on MEL - AKL, your KF points tally will go up.
You don't say what class you are in, I think that in Y, its only 20 VFF SC, x 2 as you are doing a return.
If you are premY, then maybe it would be a good idea to see about changing from VFF to KF somewhere in AKL.
If you don't have Koru access, maybe book the Emperors lounge and use their computer to see if you can change where points are credited to.
Will come back after I chase up where in AirNZ booking the FF earn part is.
Edit: maybe you already know, but, if you go into your AirNZ booking, down the bottom, it says, "Add frequent flyer membership number".
I am not game to change, or to try to change it.
But if yours is with VFF, put it to KF, thats my pov.