Vendors, taxi drivers, and street stalls rarely have change for 500,000 VND notes. Keep 10k, 20k, and 50k notes handy.
ATMs typically dispense 500k notes — break them at supermarkets, convenience stores (Circle K, GS25), or your hotel as soon as possible.
Double-check the zeros on Vietnamese dong notes; the 20,000 and 500,000 look similar in color. Confusing them is a common (and expensive) tourist mistake.