Because stir-fry's are at cooked at high heat, you want to avoid low smoke point oils - essentially the oil gets too hot and starts smoking at a low temp and begins to break down and is generally unhealthy and changes the flavor of the dish. As long as you avoid extra virgin olive oil, you can use light olive oil. The smoke point on light olive oil is in the same range as vegetable oil. So you should have no issues substituting light olive oil for vegetable oil for high heat cooking needs.