Your success will depend on how severly the seam is split. If the edges are curled-up substantially, you're probably not going to make it look pretty, but you can at least make it flat and keep it from getting worse. Try using a small trowel, or even your fingertip to get some vinyl adhesive under the edges of the seam. If the curled edges have begun to harden, you can sometimes make them more pliable by heating the area slightly with a blow dryer. After gluing and heating, weight the area overnight with a heavy book. After the seam is once again lying flat, apply the manufacturer's recommended seam sealer and allow to dry. You will probably still see the seam more than you'd like to, but at least it will resist water penetration and it won't get any worse.

If you have commercial vinyl or linoleum, you may consider heat welding the seam.