Halloween costumes with print on them generally use either a cheap silk screening process or a sticker. From the picture I can't really tell what fabric the vest is made out of, but it's probably something highly synthetic. A method you could try is take a paper bag, and place it over the print and iron it. It should remove a large amount of the image. Then you can just do as mentioned above and paint over it.
Do keep in mind when ironing that you want to use as little heat as possible. Halloween quality costumes are infamous for being made out of lower quality material, and it could possible melt it. So just be careful!