Would Mirko Vosk, Mind Drinker trigger Waste Not?
No.

“Discard” specifically means to move a card from your hand to the graveyard:


  701.8. Discard
  
  
    701.8a To discard a card, move it from its owner’s hand to that player’s graveyard.
  


Further, MTG cards are very precise with their language. Only an effect that tells you to “discard” a card would count as discarding.