How do I change the frequency of this Vibrato-Circuit?
The BJT is a phase shift oscillator and uses the three caps marked C411 to C413 to generate 180 degrees phase shift to create sinewave oscillation. You could experiment with these to alter the frequency.



This isn't the actual part of the circuit that modulates the audio but, the output should feed the modulator (the real heart of the vibrato circuit).

If you wanted more control you could use the ubiquitous 8038: -


(source: circuitstoday.com)  

It has sine, triangle and square outputs and you'll get a massive range of frequencies to use. Or maybe even consider the wien bridge oscillator. This one runs from a 5V supply: -



There's also application note 263 by TI - it's a good read and has some interesting sinewave circuits.