While you cannot exactly know what would have happened if the United States had not entered the war, one can make guesses. U.S. loans to the Allies were vital to the war as early as 1915. US. entry into the war in 1917 was important in maintaining the morale of the Allies especially France. U.S. forces were quite vital in the Allied offensives in the summer and fall of 1917.
That said, even with Russia out of the war, German civilians were suffering in late 1917 and early 1918. The British blockade of the North Sea was starting to impact German food supplies. 1917 and 1918 were bad crop years for Germany and many living under the rule of the Central Powers were starting to agitate for an end of the war. There were already strong leftist movements in German and Austria-Hungary calling for an end to the war. American involvement helped to end the war in 1918, but I am not sure that it was vital towards ensuring an Allied victory, as one can argue both sides.
Wilson's Fourteen Points created new nations in Poland, the Baltic States, and Yugoslavia based on people's rights of self-governance. Wilson called for limits of armaments and an end to secret treaties. These led to the limits on naval power brought about by naval conferences in Washington. Wilson also created the League of Nations, the precursor to the United Nations. The League was ultimately powerless as the U.S. did not join and it had no power to correct states who disobeyed its mandates. The limits on armaments were flouted by Japan, Italy, and Germany before WWII. Wilson's calls for self-determination went unheeded in colonial possessions and would lead to nationalist movements in French Indochina where the U.S. would find itself engaged fifty years later.