We booked economy air with Viking and were assigned United Airlines . When I logged in to Ua, there’s an option for us to choose the seats under economy class, then I could choose seats with extra leg room snd paid a bit more and we are fine with that. Viking had not the option of choosing this extra leg room when we booked with them , so I’m glad I could choose this on my own .
On another topic , when Viking offers free air , I noticed that it often comes with an elevated trip cost. We have to do our own research and comparison.