How does vibration motor create motion in particular direction?
Vibration does not in itself create continuous motion in one direction- the center of mass stays exactly fixed in the same place in the absence of external forces (assuming it was not moving to begin with). 

If you have feet that slip more easily in one direction than the other then vibration can cause motion. There are other options, but it's the interaction of the object with the environment that causes motion. 

Here is a paper on the subject. Link expired, new link