It's not a 28BYJ-48 (unipolar), but a Nema 17 (bipolar). I don't think the a4988 can drive unipolar motors, so I purchased this one for testing. The motor does not vibrate nor gets warm. Looks like no current is flowing at all. Maybe Vref is too low. The voltage on the motor pins on the driver board rises to 4.6 V while I'm sending commands. Starting to think it's a matter of current.