Not an expert, but I would guess that the data interface is one of two things. 1) some kind of constant variable signal that the main control board produces, like with variable duty cycle or something, or 2) actual "data" that only the control board knows how to produce and only the proprietary VFD knows how to interpret.
option one would be easy enough to do with a microcontroller or something, but option 2 would likely be far more difficult.
You might be able to gain some information by figuring out what the data line goes to, presumably an IC, and then figuring out what type of signal the IC is looking for from its datasheet.