Vietnamese has tones and isn't easy to pronounce, but even one or two words with a smile changes the mood completely. Here are the must-use phrases by situation.
Greetings & basics
The first five phrases to learn. Your pronunciation needn't be perfect, so use them often. Even just 'Cảm ơn' (thank you) earns goodwill.
The same spelling can mean something entirely different depending on the tone. If you get stuck, use Google Translate's voice/camera mode. Combined with gestures and writing numbers down, you'll be understood.
At the golf course
Phrases you'll trade with the caddie, pro shop and starter. Asking for yardage or a club and saying thanks smooths out the whole round.
At the restaurant
For ordering, paying and commenting on the food. If spicy food bothers you, remember 'Không cay' (not spicy).
Taxi & transport
Even if you mostly use Grab, these help when asking the way or to stop. 'Bao nhiêu tiền?' (how much?) is used everywhere.
Emergencies
For when you're unwell or need help. If you can't get through, immediately show staff or bystanders the translation on your phone.
If words fail you, don't hesitate to use phone translation. Vietnamese people are very forgiving of clumsy attempts and welcome the effort itself. One smile is stronger than any word.