yes the Vitz can be converted to CNG by a good mechanic/technician, main prblem is that most people have not gotten it done correctly and fail to install advances and emulators. they end up with a check engine light on the dash.
As long as you get the conversino done correctly you'll enjoy the car on CNG just as much as on petrol, and it doesn't have to be a manual since the 1.0L VVT-I engine (1SZ-FE) makes enough power for the auto to function properly.