Okay guys, I am back! with a BLAST.
First off, Naweed: You said original oil was giving better performance and you must be VERY right because you put in the wrong oil for the transmission. Toyota has been using ATF-World Standard in their auto cars since 2005.. and the 2005+ vitz is no exception. That oil doesn't need to replaced or changed.. 1st INSPECTION in the manual is at 160,000 Kilometers. and that too is inspection only.
However if you want to change and can FIND ATF-WS here, change it every 100K Kilometers or so just for kicks.
have a look at this TSB:
http://www.etimago.com/yaris/TSB/TS-TC010-07%20(World%20Standard%20Automatic%20Transmission%20Fluid%20).pdf
So everyone out there, with a Vitz 2005 or later, you don't need to change your transmission oil.. and if you want to, use ONLY the ATF-WS (AUTOMATIC TRANSMISSION FLUID - WORLD STANDARD).