I had an issue picking E+ seats on a flight from SGN-EWR (layover in HKG). UA markets this flight as a "direct" flight which means while there is a connection, the flight number doesn't change. This throws SHARES for a loop (like most things) when picking seats, especially since the aircraft type switches (737 to 747 for me). I don't know if this case applies to your trip though. I had to call UA get them to split the segments so now my PNR shows SGN-HKG and HKG-EWR (Instead of SGN-EWR) and it worked fine.