The analogy of VFDs to CRTs may be misleading in cases like this. Electrons in a CRT are ballistic: the accelerating voltage is many kilovolts, and the electrons shoot straight out of the gun striking the screen, even though the anodes are to the side, not in the screen. The accelerating voltage of a VFD is a comparatively tiny 12 to 50 volts, and electrons travel slowly toward the anodes much like in a triode. The structure of the cathode is also much different. In a CRT, the cathode is compact, and its emission is a single parameter which makes the entire screen dimmer or brighter. In a VFD, the cathodes are directly heated filaments (again more like a triode) which can have uneven emission along their length, affecting brightness unevenly. This difference is important because CRT rejuvenation is designed to affect emission; when emission returns to a normal level the tube is rejuvenated. Trying the same procedure on a VFD may affect brightness unevenly because the filament's emission is still not uniform.