I tried to PM you but PMs to you are not allowed.
I also own a 2009 Vitz 1.0 for almost 2 years and I am facing this same issue. initially I was getting around 600 - 625 km, with AC in a tank. Now it dropped to around 400 km. and this is horrifying. Went to Toyota Central Motors but no use.
Can you please advise, what I need to do?