Three things in life are inevitable: death, taxes, and Charizard Pokmon cards spiking to ludicrously high prices
In the unusual case where Unwilling Vessel has +1/+1 counters on it and then receives an amount of -1/-1 counters that causes it to die, use the total number of counters that were on Unwilling Vessel when it died, including all of the +1/+1 and -1/-1 counters, to determine the value of X
Any "as [this creature] enters" or "[this creature] enters with" abilities of the copied creature will also work
Throughout the history of Magic: The Gathering, the community has seen a wide variety of cards with mechanics being tested out in nearly every set
1 Ancient Copper Dragon Extreme Hoarders: Faern Edition A card thats as expensive in the real world as the piles of gold in its art would suggest, Ancient Copper Dragon is, unfortunately for your wallet, the best dice-rolling card in Magic by a wide margin