Here's how it works & it is smoother than I expected. The airside bus people at T2 Gate F51 were very nice.

The VN BP did not print for me on check in w UA. I got the "This is not a boarding pass" substitute.

This OR a copy of your itinerary will get you on the airside bus to T4. On entry to T4 is the VN Airlines transfer desk for check-in.