When you use one plus (r14 to r15) in your case, the tyre aspect ratio is also changed and diameter/ circumferance is kept closer to the manufactures recommendations (stock wheels), with an accepted tolerance of 3% maximum.
It's better to go for alloy 185/60 r15, instead of 185/65 r15. Also, wheel offset should be taken into consideration. Vitz 2012 offset range is between +39 to +51.