I say go for the Mequire's or Mother's vinyl conditioner. Just remember to take your time and get in all of the pleats of the seat. You may want to use a small vacuum like the Dirt Devil with the crevice tool to suck out any dry nasties that are in there, then spray it lightly with a good vinyl dressing. It may take several applications. I found that the key to vinyl dressings is to spray light coats, work it in thoroughly and then use a clean dry cloth to buff off any extra. This should yield a finish that looks good, is not greasy and will not attract dust. In addition do not forget to get all areas of the car, such as the sides of the seats, visors, a pillars etc. When you are applying the dressing just concentrate on the dressing, don't worry about overspray that gets on the windows, since they should be the last thing that you clean when detailing your car. Q-tips are good to get in the vents, since you should dress them as well, although not each time since they tend to accumulate dust as the HVAC system is used.
Good luck and let us know how it turns out.