What is the maximum frequency range of a Clapp or Vackar oscillators?
The maximum frequency is ultimately limited by the transistor you use. Or to be more precisely its frequency dependent amplification. When increasing the frequency the amplification will drop. At some point it will get so low that the transistor cannot compensate the losses anymore and thus not sustain the oscillation.

Another limit is the transistor having a frequency dependent phase shift. It is possible that the phase shift get so large that the Barkhausen condition cannot fulfilled anymore and thus oscillation becomes impossible. But this is actually a limit that can be worked around, by adjusting the impedance matching to compensate for the phase shift. 

Todays high frequency transistor allow to build oscillators well in the GHz range. So, unless you select a slow transistor, this will not be the limiting factor in your circuit.