I used SEM dye back in the early 90's on a old convertible top, it bought me another three years.. The deal is vinyl dye is actually a paint that can flex, to be a true dye it would need to soak into the material but vinyl is non-porous so nothing can soak in....Originally the vinyl is dyed as part of the chemical process when it's being made, after it's made all you can do is put something on the surface...

The more you operate the top the more surface cracks will appear in the dye, it's flexable but folding the top excedes its ability to flex...

Personally if I were finishing a restoration like you are I'd pony up for a new top so I wouldn't have to go back in a few years.. But I also understand money is tight...