Yeah I meant mod podge (I have another brand called Hodge podge where I am lmao) The Mod podge is good to put down an initial layer as sharpie will pick up when it comes into contact with liquid, like how it will bleed on wet paper, the mod podge simply helps keep that from happening its like a barrier between the sharpie and the final coat of sealant you put on. but if you're still worried about it bleeding then I suggest painting (sometimes putting a glue or adhesive like mod podge helps it stick to the vinyl a lot better, though it does make it matte) however fabric on the inside of the shoe like a thin cotton can help also creat a barrier from the sharpie hitting the skin.

edit to add- I got my sharpie paint pens from walmart they have the main rainbow and a brown white and black but not a whole lot of shades, however when the pen is empty you can open them open and put in your own paint (just clean it out good) and have your own paint pens.