Nice to hear that you got the new vitz for you. Did u buy it second hand or is it a fresh import?
hmmmm. So when you told toyota people regarding mileage what did they do? Throttle body clean etc? What did they say regarding plugs? What about airfilter?
Mileage depends on lot of things like the standard of fuel one use, how heavy foot one drives, the load one carries and driving conditions.
You can try changing your fuel station. Try not to drive heavy footed.
Frankly speaking I havnt been able to find a satisfactory workshop regarding vitz though u can go to Toyota motors potohar road 1-9 for second opinion regarding the issue.
Try posting in vitz fan club section for more expert opinion. Keep in touch if something helps in improving you mileage as it will help others in future.
best of luck.