Mbed on STM32F429VI (100 pins) instead of STM32F429ZI (144 pins)

I’m new to Mbed. I can see that boards with STM32F429ZI (144 pins) are supported directly.
Now I want to use an STM32F429VI , same device in 100 pin package. How difficult is this ?
Can I select STM32F429ZI as target and just be careful not to use missing pins or is there a better but reasonably easy way ?

Yes, only way really.
Check the reference manual for the STM32F429 you should have the pin tables for each package.