Whisk soy sauce, water, brown sugar, and rice vinegar in a bowl until the sugar dissolves.
Stir in garlic, ginger, and cornstarch until smooth, then mix in the sesame oil.
Heat the vegetable oil in a large skillet over medium-high heat until shimmering.
Add the ground beef and cook, breaking it up, until browned, 5–6 minutes. Drain excess fat if needed.
Pour in the teriyaki mixture and simmer, stirring, until thick and glossy, 2–3 minutes.
Stir in green onions and red pepper flakes. Cook 1 minute, then remove from heat and garnish with sesame seeds.
Serve the beef over warm rice.