When did OS become a LCC? It's in *A and their TATL business class seems to be highly regarded.

I assume the OP has checked for KLM flights from AMS to VIE, including the possibility of including them on the FF award ticket. With KLM, you should be able to check a bag through from your origination point with DL. Also, airlines are more likely to help with IROPs on separate tickets when they're within the same alliance or, in this case, between JV partners.