ANA used to offer their Diamond members exactly what you want, the ability to sit in Premium Economy (if seats were available at check-in) with an economy ticket, but this program ends on March 31, 2020. There's no other frequent flyer program (I'm aware of) that offers something comparable between the U.S. and Vietnam.

Some airlines, including ANA and Japan Airlines, offer a buy-up to Premium Economy on the day of departure for a fixed cost.