In the future, or after scraping off the most dirt, Vinylex is a brilliant product for all kinds of plastic/ vinyl cleaning. It also brings the initial colour back to life without looking to "shiny".
Autoglym interior Shampoo, used liberally and in small areas with a microfibre cloth does a brialliant job. Non abrasive unlike the 'magic erasers' which are quite aggressive.
as above the erasers are quite harsh to do ALL the seat, I only use them on very bad small areas. Liquid leather is also cracking, and also works fine on vinyl, brought my interior up a treat!
Ok. Modified plan of attack. General clean up with Autoglym Interior Shampoo and then hitting the stubborn bits with Doktor Power. Liquid Leather maybe as a finishing product to restore colour, depending on how the first two stages go.
liquid leather isnt a colour restorer, you have the gentle cleaner (which is what I used above) and the conditioner (which I'm not sure of the merits using on vinyl?)