Backback should be super tight on the vF-0A as Yamato really improved the hinge. In battroid mode, it should sit flush against the backplate with no floppiness.
There is a trick to it though, which is difficult to describe, especially as I don’t have the toy in front of me at the moment.
Basically, you have to angle out the part at the bottom of the backplate, then push up the hinge arm, until it locks ito place.
I’ll try to take some pics later.
I’m really impressed by the improved backpack and knee and arm swivel joints on both my VF-0A.
Graham