I plan to buy VN ticket from TPE to FRA via SGN and heard VN has few 777 aircrafts with premium Y seats.

Because VN does not sell premium Y in Taiwan, could I pay in VN's SGN desk for this upgrade when my flight will be operated by this type 777? Or there are some tips to get these seats with better seat pitch.

If I can pay for this upgrade on airport desk, how much is it about?

Thanks a lot!