actually, I think I may have solved my problem with the RIT dyes. I went to their site and they say in the faq section that their dyes DO dye plastics and vinyls. Here:
Make a concentrated dye solution in a stainless steel or glass container. Dissolve 1 package Rit powder dye in 1 pint (2 cups) hot water (or use 1/2 cup Rit liquid dye) and then mix with 1 quart hot water (140º F). Stir well. Be sure to use enough water so item will be covered and can be moved around without crowding.
Gently stir item continuously while it is in the dye solution, using metal spoon or wooden stick to lift and turn it in the dyebath, until desired color is developed. Use low heat (do not simmer) to warm the dye solution; higher temperatures may damage or melt the item.
When item reaches the desired color, rinse well in warm running water until water appears clear. Let dry on paper towels. (If color is not dark enough, simply add more dye to the solution, thoroughly dissolving powder dye first, and continue tinting the item in the hot dye solution for a longer period of time.)