Spoon the condensed milk into the base of a tall glass.
Brew a strong dark-roast coffee — a phin filter if you have one, a double espresso if you do not — and pour it over the milk.
Stir until the milk has fully dissolved into the coffee.
Fill a second glass with ice and pour the mixture over it.
Stir once and serve with a straw.
Condensed milk entered Vietnamese coffee culture because fresh dairy was scarce under French colonial supply lines, and it stayed because it is better. Robusta beans are traditional and give the bitterness the milk needs to push against.
My Bar Shelf reads your shelf and shows you every cocktail you can make from what you already own, including Vietnamese Iced Coffee.