The enduring vulnerability in being a "superpower" lies precisely in the limits of that country's actual power. During the Cold War, international affairs were primarily characterized by a stand-off between the United States and the Soviet Union, the two countries that emerged from the Second World War the most militarily powerful in the world. Much of the world, and all of Europe (with the possible exception of officially "neutral" Switzerland and Sweden) was divided largely in half, with countries aligning themselves with one of the two blocs. Even the so-called "nonaligned nations," lead by India and, after Joseph Stalin's death, Yugoslavia, tended to align themselves with one of the two blocs according to their own calculations of self-interest.
The end of the Cold War and subsequent collapse of the Soviet Union was trumpeted by many Americans as representing the emergence of a sole superpower on the world stage. The Soviet Union, a military colossus, was exposed as an economic "house of cards," with its political system having been sustained through brute force and the illusion of progress. With illusions of omnipotence now implanted in the minds of many American politicians and policymakers, U.S. leaders began speaking in terms of opportunities to reshape the world. What these leaders vastly underestimated, however, was both the limits of U.S. military power in a practical sense, and the increasing fragility of the U.S. economy as trade deficits grew and the strength of the dollar declined against other major currencies, especially the Euro.
The U.S. military defeat in Vietnam was supposed to have taught America about the limits of military power. A superpower was essentially defeated by a much smaller country, supported by the other superpower. While arguments can be made that the American loss was not preordained, the conflict did expose the limits to the U.S. ability to impose its will on other nations. The December 1979 Soviet invasion of Afghanistan should have served as a reminder of those limits. Unfortunately, it didn't, as the loss by another superpower to another weaker country (again, supported by the other superpower) demonstrated once again the practical limits of military power.
The enduring vulnerability, then, has manifested itself once again by the failure of the United States to achieve its goals in either Iraq or Afghanistan despite its far superior ability to inflict destruction on its enemies. Terrorist organizations like Lebanon's Hezbollah movement and al Qaeda have both inflicted major defeats on the United States, with the superpower now mired in a long-term effort at eliminating terrorism -- an effort with no end in sight, and that has fundamentally altered how Americans view public safety and their right to privacy.
Another closely related vulnerability involves the financial costs of being a superpower. Global commitments, for example, alliances in Europe, with Japan and South Korea, and others entail costs. Poor economic decisions by both the government and the private sector have seen the once omnipotent U.S. economy greatly weakened, and the growth of once weak China as an economic giant with increasing military capabilities now presenting a long-term challenge to the U.S. position in the world.
In closing, it actually appears as though the main enduring vulnerability of being a superpower lies in the propensity for self-delusion.