In casual eateries, raise your hand slightly and say 'em ơi' (younger person) or 'anh ơi'/'chị ơi' (older brother/sister) to get attention.
Snapping fingers or waving aggressively comes across as rude. The 'ơi' suffix is a friendly Vietnamese way to call someone — pronounce it like 'oy' with a rising tone.
Use 'em ơi' for someone clearly younger than you, 'anh ơi' for a young man, 'chị ơi' for a young woman, 'cô ơi' for an older woman, and 'chú ơi' for an older man.