With 26 year old vinyl, there's not a whole lot you can do to de-gloss it .
I've been using some Meguire's vinyl cleaning and conditioning products the last 10 years, not sold in the usual retail outlets, I get mine at an auto paint supply store .
It doesn't seem to leave a real slick surface like Armor All products .
Personally, I don't think the covering material on your seat is going to change much, no matter what you do to it .
I'd start doing some research of shops in your area that recover auto, boat interiors and see what they can do for you, if you can't find anyone that specializes in motorcycle seats .
There's always the option of replacing the cover yourself, a few members here have done this .