Hello (xin chào — 'sin chow') and thank you (cảm ơn — 'gam un') open doors. Locals light up when foreigners try.
Vietnamese is tonal, so don't worry about perfection — effort matters more than accent. A smile carries the rest.
Other useful ones: 'không' (no), 'có' (yes), 'bao nhiêu?' (how much?), 'ngon quá!' (delicious!), and 'không cay' (not spicy).