The situations facing the assembled diplomats at the Congress of Vienna in 1815 and those assembled in Paris to write the Treaty of Versailles just over 100 years later in 1919 were very similar. In 1815, Napoleon had just wreaked havoc on all of Europe by waging full-scale war on all of the great powers at once, and nearly winning. It had taken an alliance of the four greatest powers to defeat him, and Europe was left in shambles after his final defeat at Waterloo. In 1919, the Central Powers had just waged the largest war in history against the powers of Europe and had dragged other world powers, such as the United States, into the conflict. Europe was again in shambles, with astronomical death, injury, and property damage totals on all sides. However, while one treaty was realistic, and caused a lasting peace in Europe, the other was vengeful, profiteering, and caused another, much worse, World War.
Although the Napoleonic Wars, like World War I, had caused more damage than any war the assembled diplomats had ever seen, the nations represented at Vienna managed to work out a plausible theory to restore the balance of power, bring back legitimate rulers, and restrain France from causing any more damage without utterly humiliating the nation and ruining it economically. Even when, directly during the conference, Napoleon returned to France and attempted to resume aggressions against the Powers, they did not attempt to cause damage to France, only to restrain her.
All the provisions in the Treaty of Vienna were reasonable ones that worked directly towards achieving the Congress' goals. The Congress attempted a partial reconstruction of the European map. While a full return to the state Europe was in before the French Revolution was not possible or desirable, the Congress did a very good job at creating a new map that restored many legitimate rulers, as well as buffered France with states powerful enough to restrain it, such as the new state of the Netherlands, Spain (which was restored under Ferdinand VII), a more powerful Austria, as well as a German Confederation including a more powerful Prussia. All the nations present at the Congress were benefited in some way, but all the nations compromised their interests to some extent to ensure that the balance of power was kept intact.
Aside from the new European map, the other provisions made by the Congress were also fair, and helped Europe achieve and maintain peace, without humiliating the French or wheting their taste for revenge. The Congress gave France a fair amount of war reparations to pay, which the French accepted without objection. They also put foreign troops in France's buffer zones to ensure that France would not resume aggressions against the rest of Europe. These mild punishments for France were due in large part to the fact that France's diplomat, Talleyrand, was allowed to attend the Congress and was given a full vote in the proceedings.
These changes to Europe as a whole and mild restrictions on France are what made the Congress of Vienna a success. None of the treaty's provisions devestated the France of her economy. In fact, they allowed France to return to a normal, functioning economic state very quickly, allowing the balance of power to be restored and ensuring a lasting period of peace until the Crimean War.
On the opposite side of the coin was the Treaty of Versailles, signed in 1919. This treaty was completely unfair to the Germans, and stung their sense of pride so badly that it was easy for Adolf Hitler, a man preaching German superiority, to come into power and lead Germany into another destructive war.
The Treaty was very destructive to the German military, but in particular to Germany's economy. It forced Germany to rapidly reduce the size of its military to several units totaling only 100,000 men. Germany also had to reduce its navy to a handful of ships, and eliminate any airforce. In addition, there were myriad other restrictions on how many of each type of military unit could exist, how much weaponry and ammunition each unit could have, and where certain units could be stationed. Specifically, no military units could be stationed, and no fortifications could be in place in the large area known as the Rhineland.
Much more devestating, however, were the restrictions on Germany's economy and the war reparations she was forced to pay. Effective as soon as the treaty was signed, many restrictions were placed on the German economy. Most importantly, all Germany's foreign posessions were taken away, along with any private or government property in them. In addition, Germany's economic capabilities at home were drastically restricted, and she was forced to liquidate much capital immedietly in order to pay the war reparations of 33 billion gold marks, an amount which the Allies could increase whenever they saw fit.
All these measures against Germany were written to take into account only the immediate economic needs of the nations of the Alliance, not the long term safety and prosperity of the world as a whole. Germany did not even have a diplomat present while the treaty was written. When the treaty was first presented, the Kaiser refused to sign it. However, the Allies threatened more hostilities, and germany was forced to accept the Treaty unconditionally.
The Treaty of Vienna and the Treaty of Versailles were two diametrically opposed documents. The treaty arrived at by the Congress of Vienna was a totally fair one, which was written only with the interests of the European community as a whole, even including the defeated French. The delegates at Vienna wanted only to ensure that there would be peace in Europe and no further hostilities from France, not to humiliate France, take revenge on her, or use their position of authority over France for their own economic benefit. The motives of the delegates at Versailles were direct opposites to those of the delegates at Vienna. At Versailles, the delegates wanted to completely crush Germany, make sure she could never be a military threat again by crippling her economy and military, and use her for their own economic benefit, without regard to the future or the safety of the world at large. This is the difference between these two treaties - the motives of their delegates. And this is what caused the Treaty of Vienna to be a complete success and the Treaty of Versailles to be a miserable failure.