You first need to identify whether it is the display itself which is at fault, or the circuitry which drives it. Dim displays fall, I would say, roughly 50 / 50 into low drive volts, and worn out. Often, the drive volts are derived from a voltage multiplier, and it is very common for the input capacitor to dry out and go low value, which leads to a low supply. This is typically about -30v DC when correct. The filament supply, which you can normally measure at about 1.5 to 3v AC, is usually floated on top of the -30v, so that is a good place to measure it. Most VFDs have a pair of pins at either end of the main row. Measure across these to check the AC filament supply, and from either one or the other to ground, to measure the negative supply. If these are all in order, then the display is worn out. Often, when it is worn out, there will be patches that are brighter than the rest. You may be able to squeeze a bit more life out of it by boosting the filament supply a tad, but if you go too far, the filament wires start to glow a little, and may become visible. Things may be a little different with a large 'commercial' display like this, but the basic principles should be the same. Do you have schematics for it so that you can see how the drive voltages are derived ?