Make the sweet and sour sauce first. In a small bowl combine the brown sugar, ketchup, soy sauce, rice vinegar, water, corn starch, and ginger (if using). Stir until combined.
Heat the vegetable oil in a large skillet or wok over medium-high heat. Mince the garlic, add it to the skillet and saute for 1-2 minutes or just until it is slightly softened.
Add the frozen vegetables to the skillet or wok and saute just until warmed through (1-3 minutes depending on whether the vegetables were thawed or frozen).
Give the sweet and sour sauce a quick stir to re-suspend the cornstarch and then pour it into the skillet. Allow the mixture to come up to a simmer. As soon as it simmers it will thicken. Make sure everything is heated through and then serve.