The term 'Vessel for Jack and Jill' is a reference to the nursery rhyme 'Jack and Jill', which tells the story of two children who go up a hill to fetch a PAIL of water. The solution to this crossword is 'PAIL', which is a four-letter word meaning a wide-mouthed container, usually made of metal or plastic, used for carrying liquids. A PAIL is the perfect vessel for Jack and Jill to fetch their water from the hill.