The big problem is that vibration motors do not have a constant amplitude. Both the vibration frequency and amplitude are dependant upon the speed of the motor (and therefore the input voltage):

Vib Freq = Motor Speed (RPM) / 60

and

F = m * r * w^2

where F is the force of the motor, m is the mass of the eccentric mass, r is the eccentricity of the mass, and w is the angular velocity.

So you can't adjust the frequency without affecting the amplitude, and vice-versa. Linear Resonant Actuators separate the vibration amplitude and vibration frequency, however only work at a very specific resonant frequency. See this blog post:

http://www.precisionmicrodrives.com/tech-blog/2012/05/11/driving-lras-from-audio-signals-and-musicOn the vibration motor's datasheets you can see how the amplitude and frequency adjust with the input voltage.