A Magic: the Gathering strategy concept
Virtual card advantage is a type of card advantage that is not discussed in Saige's fine writeup of the term as it is used in Magic: The Gathering strategy discussions. The principle behind it is the same as the principle behind card quality advantage, but is generally used in the opposite sense as one uses quality advantage.
Saige defines card quality advantage as "...being able to get the better cards from your deck, or being able to get a hold of a card you need for a situation when you need it." Note that this refers only to improving one's own draw. Virtual card advantage, on the other hand, is defined as having cards your opponent draws not as capable of improving their position as much as the cards you draw. Typically in implementation, it means making a large subset of your opponent's deck useless. Think of equal size armies, with one side having ten times the weapons as the other. The two sides certainly have equal numbers of fighters, but since most of one side is impotent, the battle will be very lopsided. Likewise, when one has obtained virtual card advantage, one might have just as many cards available as one's opponent does, but since some of them can't affect the game as much, those draws were wasted. Thus, one has "virtually" more cards than the opponent.
A very common way this is manifested in a game is by playing a card that can be tapped (used) once per turn to kill a certain kind of creature. If an opponent plays many of that variety of creature, that opponent can either attempt to overwhelm it by playing all of them quickly, ending the game before the creature killer can be used to gain enough card advatange to take control of the game, or that player can hold the creatures in hand, hoping to draw into a way to deal with the situation. In the latter case, one gets virtual card advantage for every creature card the opponent draws (and keeps in hand) that would get killed if it were played.
The card Moat (Creatures without flying can't attack) stands as the classical example, both by having been around since near the beginning of the game as well as possibly generating an obscene amount of virtual card advantage. The card stays in play until your opponent gets a specific type of card (enchantment removal) which might not even be in their deck, and prevents a very large class of creatures from attacking. While similar to the above in that the opponent's creatures are useless, there is no choice in how to deal with it. One can't attempt to overburden the killing machine, but can only sit in front the wall that was erected and hope to have an alternate victory condition. Facing down a deck using Moat well without being prepared for it can make almost every card in a deck useless. Only cards that can remove or get around the barrier are useful at all, while potentially every card in the same deck as the Moat remains useful.
Besides making cards one's opponent might play less useful, virtual card advantage also can be used to describe situations where the cards one plays make the opponent completely unable to play most of the cards in that player's deck. While one way is to play cards that deny players mana, there's also another very simple way: win the game before the opponent can play them. In this case, it is really a time advantage that creates the a card advantage, but it is this aspect of fast decks that require slower decks to play some inexpensive ways of dealing with cheap threats. Playing only expensive spells that can't be used until the game is lost means giving the faster deck the ultimate in virtual card advantage: they've made every card you draw completely useless.