Are you talking about mileage with AC or without AC?
Vitz will do about 13-14 Km /Litre in city like Karachi with AC on almost all the time, without AC, it might exceed 15 KM/Litres. It also depends a lot on factors like you do long drives or short stints, because O2 sensor takes time to heat up to working temperature every time you start and until that time, engine uses more fuel. You can see that your driving habits can change mileage by almost 3 Kms a litre or even more
Secondly, if sensors are a problem, you should get a "Check Engine" light. If it is not there, there is a little chance that there is problem with sensors. Also, the type of petrol you are using also makes a difference. Try using a mixture of Hi-octane and super (about 50 50) and check the mileage, please post it here.