The SGN-PNH flight is short and thus pretty painless. Nothing to write home about, mind you, but since it's only 45 minutes, it shouldn't make a difference.

The problem, IIRC, is that the flight times don't allow for same-day transits (at least, that was the case a couple of years ago), and so you would need a visa, need to stay in SGN overnight, etc.

Are you stuck with UA for some reason? Why not fly TG?

- Michael