Yes most airlines discriminate. However just because inventory shows doesn't mean an airline has to confirm a reservation issued by another airline. They can choose to refuse to allow it to be reserved. Also there's nothing to say that the phantom VN inventory isn't phantom, but just a matter of VN inventory not updating in other airlines systems.

There could also be legal restrictions that prevent VN from offering a certain combination of flights. Thus while there's inventory, if they're not allowed to sell a set of flights between two countries the VN system would block it.