You probably could get vactrols to do… something. Put one in parallel with the variable resistor and the resistance you end up with would be something lower than either of the two separately, which means the vactrol would be largely ineffective if the pot is near one end (low resistance) but a lot if the pot is near the other end (high resistance). If it were in series it would add resistance, so would have a lot of effect if the pot is at low resistance but very little if the pot were at high resistance.
Another option would be to use the vactrol in place of the pot, and then use a pot to attenuate the vactrol CV. You could normal the CV input to something like +10 V and then with no CV plugged in, changing the pot would manually change the frequency (while with something plugged in, it would reduce the maximum effect of the CV). But you’d probably need it to be something like a 500Ω pot at most.
You’d need six vactrols if you wanted to control all six oscillators, of course. Maybe just controlling two oscillators would be good enough? And whether sweeping the oscillator frequencies with CVs is actually a useful thing to do with this circuit is I think not obvious (at least if you’re using it for percussion. Since there’s no envelope generator built in you could also use it for a noisy drone, in which case voltage control would make more sense.) Nor is it obvious which if any of the above three approaches would give you a useful setup with a useful range. Breadboarding before building would be a good idea.