Elvis never drank alcohol. Alcohol affects your ability to sing well. You should not drink at all if you want to sing at a professional level and if you do then definitely son't drink on the day, or even the day before a performance. If you did drink some alcohol, exercise it out of your system as soon as you can and drink a lot of water.
Elvis can be seen drinking water at rehearsals and during his performances. He knew about hydrating his vocal chords.
Right before going on stage Elvis would clear his lungs with an inhaler. The proper solution to put in an inhaler is food grade hydrogen peroxide 3%.