I have Suzuki Khyber 99 Model. I have took it to Murree several times with 5 Passengers (All adult) on CNG. No problem at all. It climbed easily with all that load on CNG. But if you are going towards Ayubia or Nuthiagali then you should drive the car on petrol.
I think with a well maintained Vitz, you will also experience the same, if its manual.