Hi,
a lot of informations missing:
* Is the micrcontroller supplied with the 5V_Out of the VFD? If not, how?
* Is VFD_GND connected with digipot_GND. If possible: do this.
* What about the GND connection to the microcontroller?
* How is the digipot supplied?
* Did you install all recommended power supply capacitors?
Use short, twisted and/or shielded cable between VFD and digipot. Use seperate wires for "B" and "GND".
Use short cable between microcontroller an digipot.
***
Noise pickup can be via air, or conductive via cabling.
Pickup is increased with high impedance signals.
Pickup is increased without proper GND wiring.
Influence of noise pickup can be reduced by proper filtering.
Thus a detailed assistance is not possible with your given information.
We don´t want to design a new circuit for you.... we just want to improve your circuit. Therefore you have to show us. Complete, with sufficient description.
Klaus