A buffing compound with a wax applied with a professional buffer will remove most ghosting.
There are 3 types of ghosting though. ghosting from the paint being burnished from sand/wind/rain/snow/birdpoo/whatever ... the vinyl will act like a mask so there will be a glossy lettering left over ... buffing will fix this or atleast throwing a new clear coat on the vehicle. There are 2 types of discoloration I have come across, fading and staining. Fading generally occurs when the paint oxidizes or when it reacts to the sun and literally becomes whiter. Obviously, with oxidation it can be buffed and color fade will be reduced if not goes away, true fading needs repainting. Oh and just to make sure I mention this ... white can fade, as true white paint is impossible to get as it's still particles of pigment in a suspended binder ... batches can be subtle off whites that can fade to a white depending on what/where/when/etc they get the pigments. Staining. I have seen letters stain the paint generally when someone (mainly truckers) take their vehicles through the acid washers. It kills vinyl and the acid will react with the adhesive and stain the paint with an orange to purple hue, I've seen this more with metallic flake vinyls than anything else ... but it still happens. I don't know how to fix it, but I suspect that sun light will bleach the discoloration out since most of the discoloration comes in the heavy flake areas of the vinyl.