The VS Seeker is a very special item that allows players to rematch certain trainers found within the overworld of the Kanto Region (not counting Gym Leaders or the Elite Four). The VS Seeker aids players in further raising their Pokemon Teams, with the Pokemon Teams of these opposing trainers changing as one would progress further in the Main Story. Unlike other items, the VS Seeker is pretty easy to obtain. In order to do so, all you must do is speak with the trainer in the Vermilion City Pokemon Center. Simple enough!