Good and evil exist in balance, much like yin-yang. There are equal parts of a whole, and must inevitably continue to be in balance. In stories of long-term literature, most readily seen in comic books and manga/anime, a character of heroic proportions comes to be, often in media res. The character is usually expounded on in latter entries in the series, usually with a back story that explains an origin for the character.
As a result of this doer of good, the balance is swayed to good, making a void on the side of evil. What follows the existence of the hero doing good acts in protection against typical evil is the creation of the atypical bad guy. This individual acts as a stronger-than-normal force in the bad balance, and thus helps aid the good/bad dichotomy.
Some possible reasons for the vortex creation:
1. Logic: Where do all these thugs come from? If the main character doesn't face a real threat, he'll run out of mooks eventually, and there's no story. Even for mooks to pose a threat, they usually need someone better pulling their strings.
2. Tension: Where's the challenge? I mean, sure part of the reason superman is cool is knowing he;ll win and watching him thrash the baddies. But the reason he's died a couple times is to prove that he can be stopped, if the baddies get their game together. Comic readers know that Superman can die, and that allows for a certain amount of tension. Sure he'll probably still win. but, you never know.
3. Achievement: Beating a 10-year old at chess is nothing to brag about, unless you're 9. Sherlock Holmes figuring out a normal guy's plan is something similar, so he needed someone to actually face down. Enter Moriarty. A challenge, someone he (and the readers) can feel good about defeating.
Villain Vortex, then, is the possible trope that examines the balance of good and evil in long-term literary works. A good older example is the Sherlock Holmes series, which embodies an exceedingly wise man who solves the previously unsolvable cases. He is then met with equal power by Professor Moriarty, who is malevolent character with a great deal of influence in the world of evil...
Slews of more recent examples exist in comic books and, especially, in the films upon which they are based. The first Spider-Man movie had the existence of Spider-man explained, then shortly after his existence the Green Goblin appears to balance the good/bad scale. The second and third movies further embody a single entity source of conflict (with the third film creating a dual-source combined into one, though the evil counterbalance is swayed by the creation of the suddenly-good Hobgoblin--and further by the death of Hobgoblin and "retirement" of the Sandman, with only Spidey and Venom on the good/bad scale).
X-Men films create the same idea of balance, though using "teams" of good to balance out equal parts evil...
This is something of an application of the Sorting Algorithm Of Evil, or a justification for it. It can be brought up as a cosmic law, i.e. the Balance Between Good And Evil, or just a sort of natural result of human nature, wherein the presence of a do-gooder inspires the evil-minded to greater heights.
This is used in The Dark Knight Saga to explain the existence of supervillains. At the end of the first film, Gordon puts it in terms of escalation:
"We start carrying semi automatics, they buy automatics, we start wearing Kevlar, they buy armor piercing rounds, and you're wearing a mask and jumping off rooftops. take this guy: armed robbery, double homicide. Got a taste for the theatrical, like you. Leaves a calling card.''
In The Legend Of Zelda, this turns out to be the case, as explained in Skyward Sword. Link has a curse on him, so that every incarnation of his spirit will be faced with a great evil to fight.