Using a digital signal to turn on and off a vibrating motor
Whether you need a transistor buffer or not will depend on the current capability of the IOIO-OTG board, and the current requirements of your motor.

A quick look through some of the IOIO_OTG documentation didn't reveal any IO current spec, but a photo of the board just shows one chip - presumably the PIC microcontroller, so I would assume the board can only safely drive a 20 mA load.  I would expect any motor to require more than 20 mA so you will almost certainly need a transistor buffer to control it.