"Dim but working" entire display can usually be brightened up by boosting the filament current a little, to compensate for old tired emissive coatings on the filaments. This will also speed up the death of the filaments, but if it makes the unusable display usable again for a while, then it's worth it. If it is just a few segments that are dim, then that is the phosphors that are worn out, and nothing can be done for them.
I like to see these excellent Keithley instruments fixed. So, CJay, to compensate you for giving away your VFD to a good cause, I'll offer you a 20x2 character module for free, just pay postage (same as your offer to azer). This module has a controller and 8 bit microcontroller interface so it is more suitable to projects than a bare VFD glass.