If the existing bathroom floor is timber you may need to strengthen it first by screwing plywood (18 or preferably 25mm) down first. Then treating the plywood with one coat of PVA adhesive diluted at 50/50 with water, then a further 2 coats applied neat. Then lay the tiles on flexible adhesive. Timber bathroom floors move about a great deal and any movement under ceramic or quarry tiles leads to cracking of joints and tiles very quickly indeed. If the vinyl is laid on concrete or screed then as long as the tiles are stuck well down you can paint them with 2 coats of PVA adhesive (See our project on PVA in the projects section by clicking on the link above) and then use a powdered, cement based quarry tiles adhesive that is very easily mixed by hand. Your local builders merchants or flooring company will stock this.