DO NOT use constant current mode of the power supply. The filaments have a positive temperature coefficient, meaning that they increase in resistance with temperature, and they increase in temperature a lot. If using constant current, you can get into a thermal runaway situation where the power goes up (due to resistance going up), which increase temperature, which increases resistance, which increases power & temperature more... you get the idea. Adjust the voltage instead. This really is very important.
The idea is to use heat clear away stuff (whatever it is) that has coated the filaments and is preventing them from emitting electrons efficiently. It may or may not work. Also note that, like all other types of vacuum tubes, the filaments in a VFD have a limited lifetime.
Also effective is simply increasing the power used to heat the filaments in normal operation. This will increase electron emission, increasing display brightness. It will also reduce filament lifetime, maybe drastically. As long as you don't exceed the point where the filaments just barely glow visibly (in a dark room), then you are probably OK, at least for a while. If it means getting more useful life from a display that is unusable today, then it may be worth it.