Is it reliable to exceed the rated voltage for the 1638 SHAFTLESS VIBRATION MOTOR 10X2.0? I have an existing circuit that drives the motor from a lithium ion battery so the voltage can be 3.3 volts to 4.2 volts. The motor is on for 500 milli-seconds or less.
That should generally be fine. Running the vibration motor above its recommended voltage range should increase the vibration speed at the expense of its operating lifetime. 4.2V is still close to the recommended range, so I would not expect that trade-off to be consequential. If you want to try to maximize the operating lifetime then you might consider powering the motor through an appropriate MOSFET that you can control with a PWM signal so you can limit the duty cycle.