How to clean wood stain off of vinyl porch?
The exact fix depends on the stain, but try a few common solvents like rubbing alcohol, wd40, and acetone (nail polish remover). Be sure to try them first in an inconspicuous non-stained area with a cotton swap, as some solvents can etch some plastics, e.g. acetone and ABS.
You might also try steaming through a towel; that can remove some things solvents have trouble with.
If all else fails, you might be able to carefully bleach the stains to reduce their appearance. Regular bleach or the hydrogen peroxide gel packet from a cheap blonde hair-coloring kit would be my choices for that.