Take Vivillion to a Move Relearner.
Dendemille Town in XY and and West of Fallabour Town in ORAS
Give him a Heart Scale and Vivillion will relearn Sleep Powder as Vivillion gets Sleep Powder at lv.1.
Vivillion's Moves Page
You go to the move relearner, and with a heart scale, it should be able to be taught Sleep Powder.
Hope I helped!