I tend to agree with james_s, it is worn out. I would have suggested the rejuvenation process of heating the cathodes to clean off buildup, and/or boosting filament current a little to improve emission. Since you can see the faint red glow of the cathodes, they have been run hot all their life and are probably simply worn out. The oxide coatings which emit electrons don't last forever. On a healthy VFD, the cathodes usually do not run so hot as to visibly glow, even in dark conditions.
As a temporary fix, I'd suggest boosting filament current until the display is usable. This will speed up the aging process so don't expect to double its life or anything.