I was frustrated with a 35 year old vinyl roof with a grain. Tried everything.
What worked for me:
1. SOAK w/vinyl cleaner (Tutle Wax, Meguiers,etc)
2. Rub in cleaner with Mr. Clean Magic Sponge
3. Get into smallest pores with 0000 steel wool lightly
4. Rinse w/clean water and very little detergent.

I do not not know if you can see the difference in the pic but it is dramatic in person.