The most common source of reduced fuel mileage is low tire pressure. However, there are MANY other things that could contribute to worse than normal fuel economy. The VSA light on in your car means there is a malfunction with the "Vehicle Stability Assist" system. I would suggest to start with fixing things you know are wrong with your vehicle like whatever is causing this VSA light.
For future reference here is how to do a proper fuel consumption test. Pick a certain pump at a certain gas station. Use this same pump for every test you want to conduct. Fill your vehicle till the pump shuts off automatically. Don't top off fuel after this point. Note your mileage (or kilometers) and drive till its time to refuel again. Note your vehicle's mileage when you refuel at the previously used gas pump. Finally, note the volume of gas the pump reads after it shuts off automatically and use this formula:
(end mileage - start mileage)/ volume refilled = fuel economy
Note: Don't buy into these "mechanic in a can" products like gas station fuel system cleaner. There is a time and place for similar products on certain vehicles but not all. Ask your mechanic before potentially wasting your money on such products.