There's a great selection at the convenience stores over there. Apart from cheap street food, you can pick up your standard sodas, beer, sandwiches, chips, and all the "normal" snack stuff you'd expect. Circle K is the one you'll see on every second corner — it's actually the original foreign chain in Vietnam, opening its first store back in 2008, and now has more locations nationwide than any other. 7-Eleven turned up later, in 2017, but is still mostly confined to Ho Chi Minh City.
Here's what some of it will cost you, all converted to NZD:
You can find many of these cheaper again off the street vendors, but for a cold drink or a snack when you need one, the convenience stores are everywhere, open 24/7, and hard to beat.