For this, I needed two items: a toothbrush and non-acetone nail polish remover.
It was a simple procedure: I took off their clothes and scrubbed the toes with nail polish remover, using the toothbrush.
Here is Julie's "before" picture.
When I was done, I washed their feet with soap and water, just to be careful.
Non-acetone nail polish is not supposed to hurt the vinyl, but I didn't want to take chances by leaving it on the vinyl any longer than needed.
This was super, super easy and I am very pleased with the results!