If, at the time you wrote this letter, you thought you were actually giving back the vows, then, in that case, you are no longer ordained. It would all be finished then. Once you generate the thought that the vows are gone, they’re gone. You don’t have them after that. Holding vows depends on the decision made in one’s mind. For example, when the vows are initially taken, you have to take them with a strong, decisive mind, and also you must believe that you’ve received them.
If you didn’t write the letter with the thought that you were giving the vows back at that time, you can give them back to the geshe at the center near your home.