I am not sure what would be best to use on stubborn vinyl dirt on my grips.

If you have one lying around use a clay bar.

Also do the gearheads a favor and place your year and trim level of your Bonneville in your signature. It helps them to give you advise on questions.

you can also try scratch X

Tufstuff, let it soak in for 30 seconds & use a hard brush. (like the one you would use on your tires) then wipe ti clean with a wet cloth. Mine had black spots where you grab the handle & nothing else would clean it. Now they look like new.