I use straight bleach. wear gloves, get a rag saturated in bleach and wipe it on. Keep it wet with the rage while letting it soak for five minutes or so. Then use a green scrubby pad or #0000 Steel wool and 90% of the time it will come nice and clean. If not, you can always sand them. Yep, sand them with a fairly fine sand paper. Either way, when you are done, put a good vinyl protectant on them to bring back the luster and bring out the color.
You can try a regular cleaner first, but if that doesn't work to your liking, then try the bleach and last resort, grab the sand paper.