So if i get stand and fight by using Vilya, can i put as an example Beorn from my discard pile without any cost? It seems right to me but i'm not entirely shure about this.

You cannot. X is considered to be zero in this case.

(1.40) The letter X Unless specified by a card effect, or granted player choice, the letter "X" is equal to 0.

Nope. X = 0

https://community.fantasyflightgames.com/topic/135580-vilya-x-cost-cards/

So sad. Well thanks for quick replies.