If you manage to put a hero from the discard pile onto the top of your deck, can you put him in play with Vilya?

From the Core Rules:

"Note that hero cards that are defeated are placed in their owner’s discard pile. When resolving effects that move cards from a player’s discard pile to his hand or deck, hero cards in the discard pile are ignored, as hero cards cannot move to a player’s hand or deck."

So sadly, no.

I guess the exact answer would be that if you have managed to put the dead hero into your deck, then Vilya would work, but since there is no way to get it back into your deck it will never actually happen. If a hero got sent to your deck by e.g. Lost And Alone, I'd see no reason why Vilya wouldn't be able to bring them into play.

Oh yeah, I forgot that some Treachery cards put Heroes into your deck sometimes.