Hi once again,
I couldn't find any data for that VFD, so the only thing to do is to try a couple of things, if you think that they are worth the risk.
You can use a DC power supply, connected directly across FLA1 and FLA2, and, starting from 0 volts, slowly raise the voltage until the filament glows visibly red. Leave it like that for about 60 seconds, remove the PSU, and power the unit. With luck this MAY rejuvenate the filament. But if you put too much voltage across it you could burn it out!
The other thing you could try is to put an extra turn or two on the transformer (if there is space for winding it around the outside of the existing windings, of course), checking that it only gives about an extra 10% voltage (0.35V-ish AC), and connecting it in phase, in series with the existing filament supply. Again, it may or may not work!
Regards,
Sarah.