Place the vinegar, oil, and sugar in a bowl and mix well. In a mixing bowl, add watercress, onions, tomato and dress with vinaigrette. Set aside.
In a hot wok or pan, add oil and cook garlic until fragrant.
Turn heat to high adding beef and stir fry for 1 minute until browned, then season with a pinch of salt and 1/2 teaspoon pepper.
Add the beef to the salad bowl then dress with nuoc cham. Mix well, then transfer to a plate and garnish with chile.