So when playing MTG I had down Vedalken Orrery, and I was playing a creature and my opponent counters my creature. In response I play Duress, as Vedalken allows you to cast spells as if they have flash, which is basically instant speed. Can I Duress his Counterspell?
No. Once a spell has been cast, it's no longer in hand but on the stack, so you can't make your opponent discard it any more.
As part of the spellcasting process, the card moves from the hand to the stack; the stack is a zone like the hand, battlefield, etc.. Once a spell has been successfully cast, it's no longer in hand and cannot be discarded any more, since the definition of "discard" requires the card to be in hand. You would have to play Duress before your opponent plays the Counterspell.
400.1. A zone is a place where objects can be during a game. There are normally seven zones: library, hand, battlefield, graveyard, stack, exile, and command.
601.2. To cast a spell is to take it from where it is (usually the hand), put it on the stack, and pay its costs, so that it will eventually resolve and have its effect. [..]
701.8a To discard a card, move it from its owner’s hand to that player’s graveyard. “Discard.”
1To Duress the Counterspell, would it not be "You would have to play & resolve Duress before your opponent plays the Counterspell"? If you just play Duress then the opponent can play Counterspell in response while Duress is still unresolved on the stack.– MT0Mar 2 at 10:02
1@MT0 If you're trying to Duress a Counterspell, and your opponent uses that Counterspell on your Duress ... problem solved (although you wouldn't get to see their hand). Mar 2 at 10:19
@NotThatGuy Yes, it solves the XY-problem of getting rid of the Counterspell from the opponent's hand but doesn't solve the question the OP asked of "Can I duress his Counterspell?"; for that you need to play & resolve the Duress while the Counterspell is still in their hand.– MT0Mar 2 at 10:26
1The core of the question is whether or not Flash Duress is an improvised Counterspell, whether you can hold out on Duress until you know you need it to discard a Counterspell so you don't potentially waste it for lack of valid cards to discard. Because I believe OP understands how casting and resolving works otherwise, I left out that clarification about resolving. And yes, as mentioned, the implication of "play Duress before your opponent plays Counterspell" is "play Duress before your opponent has a valuable target to play Counterspell against". Mar 2 at 10:46
1@GendoIkari that's not quite correct. If you have two Counterspells in hand and only UU available, then not countering the Duress means you can counter whatever opponent plays next. But it's an interesting point that much of the time it's best to play Counterspell. Reminds me of this strategy article, which is high-level enough that it's not easily understood, but makes perfect sense once you understand it: strategy.channelfireball.com/all-strategy/mtg/…– AllureMar 7 at 1:39