Hi,
so exactly what pin are you using.
The page you posted showed that any of four pins can be used PoKeys pins 18,20,21,22 and according to the attached pic Pokeys pin20 is DIO33 type
with a max output of only 4mA.
Another pic shows 0-10V analogue output as isolated but no detail as to what drives the pin...not much help.
It would appear in either case that the signal needs to be buffered before it can drive your VFD input. I am a little surprised that PoKeys didn't publish the circuit diagram
of the output drive of the pin, means you have to guess.
Craig