In a bowl, toss the beef with 1/2 tablespoon of vegetable oil, 1 tablespoon of garlic, 2 teaspoons of sugar and the fish sauce; season with pepper. Let stand.
In a bowl, toss the onion and vinegar. In another bowl, mix the soy sauce with the olive oil and remaining 1/2 teaspoon of sugar. Season with salt and pepper.
In a wok, heat the remaining 2 tablespoons of vegetable oil over high heat until smoking. Add the remaining 1 tablespoon of garlic and 1/4 teaspoon of salt; cook until golden, about 30 seconds. Add half of the meat and cook for 3 minutes without stirring; turn and cook 1 minute longer. Transfer to a plate; cook the second batch. Return the meat to the wok and stir-fry for 30 seconds.
On a large platter, toss the pea shoots with the onion and the soy dressing. Spoon the beef on top and serve.