The most important thing here is to make sure of which problem it is. Prolapsed vents in newly hatched chicks are a bit different than in older chickens, who get them from constipation or from egg binding.
Sometimes they are a birth defect. Some may outgrow it or some may have other defects. If constipation is a problem you can try treating that with a few drops of molasses or stimulate the vent gently with a little olive oil and a QTip. Hydrocortisone cream applied to the prolapsed vent may help to shrink it, so that it may go back in.