Assign a Soft Knob or modwheel to the value you want. The destinations you would want are only FreqShifter Freq and FreqShifter Mix (for the matrix and for the soft knobs). Or you can select Others in the effects section, effects edit until you get to the freq shifter and change them using the soft knobs.
In case of the Vowel filter frequency you get a % value and a phonetic vowel