where is the vent valve on this car.

My service information shows the vent valve as mounted on the vapor canister on your 1998 Ford Contour. The vapor canister is located on the under side, mounted on a carrier bracket. I would rate the difficulty at about a 5 mostly because of the need to work on the underside of the vehicle. I would suggest you take a look under your Contour and see if you can locate the vapor canister. You can then make your decision about the difficulty of access to this part.