The easiest approach would be to use an ADC to measure the voltage over the potentiometer and change the DAC accordingly. Since the DAC in a PSoC3 delivers a voltage referenced to GND I see no other chance to change its output.
Why do you want to change the VDAC reference, is this for shifting offset or for changing gain or for both?
Depending on what you want, you could probably configure an OPAMP component to do the job.
If you are looking for a multiplying DAC you could use DAC trim register to effect a
Or take a look at this thread -