The loss of plasticizers is a natural phenomenon but can be accelerated by UV, Infrared, airborne pollutants or improper cleaners or protectors. The loss can be retarded and the useful life of vinyl extended by application of a protector that restores the plasticizers. One such protector is Vinyl Guard. Regular use has been proven to double the life of vinyl. This applies to vinyl seats as well as tops.
My suggestion is to try washing the top well with EZ Vinyl Cleaner. Rinse well and let dry. Then, apply 2 coats of Vinyl Guard. You should feel an immediate difference. It will be slight at first but re-cleaning and application every 2 to 3 months should increase the suppleness and flexibility. if this does not work there are no other options and you will have to replace the top.