The API does not let you control the speed of the motor, you can only set it ON/OFF.
You send the duration you want the motor to stay ON. After that time the motor turns OFF automatically.
For a better effect you have to use PWM/duty-cicle.
Here's a helper class to do that. https://github.com/tainicom/XNALibrary/blob/master/Source/Devices/Vibrator.cs
You have to call Vibrator.Current.Update() on every frame.
Set Vibrator.Current.Dumping somewhere between 0.7-0.95 so it will stop after a while.
Call Vibrator.Current.Vibe(power) whenever you have a collision, explosion, bullet-hit, button tap, etc. power is between 0.0 - 1.0.
Note: (1) This kind of feed-back will mess with the accelerometer readings.
(2) Some devices have weak motors, while on others are more noticeable. Let the user adjust Vibrator.Current.MasterPower from the settings.