delay time all depends on how fast you need it to move.

Please do some simple troubleshooting for us. Replace the motor with LED's. then you will be able to see if all 4 lines are stepping. Vibrating can also come from a missed step or 2. Also could you please apply 5V to the input of the ULN, 1 at a time and see if the motor moves?

This problem really needs to be isolated to hardware, software, or both. At first glance, it seems like your program should work. I don't know if you will get your desired function, but the motor should move.