It may be down in the vinyl and very hard to remove.
Try a paste with Oxyclean or other oxygenated bleach, not chlorine bleach. Let it sit there for 24 hours, keeping plastic wrap on it, to stay wet.
You can try Krud Cutter.
Heated up white vinegar. Cover with plastic and let sit a few hours.
Try Scrubbing Bubbles, the bathroom cleaner. I used it to lift out all kinds of stuff over the years from vinyl flooring. Leave it on for about 10 minutes before scrubbing.