If you are allowing the user to turn a knob to set the time, you will need to measure its position and also display its current position to the user.
You will need an LCD display, or 8 segment displays etc, and look into using a rotary encoder for the knob. you can specify the min and max values, 0-30, and then multiply its position by 60,000 to give you the minutes in milliseconds, and store it in a variable for comparison.
Rather than using a delay(xxxxx) I would be inclined to take a reading from millis() which is how many milliseconds have passed since the sketch has been running and then using an if condition to see if the time has passed.
if(millis>duration)
{
//Turn on the vibration motor
}
You will also need a section of code which detects each change of the rotary encoder, and on each change it resets the millis back to zero.
if you are incorporating a start button etc, then encase the lot in a for loop, with the button press increasing an integer value which triggers the condition of the loop
There are some easy to use examples on the arduino IDE which will show you how to use an LCD sreen.