For most of American history, wars have tended to produce Presidents. Washington, Jackson, Grant, Hayes, Garfield, Arthur and Eisenhower were all generals during war, and veterans of all of the "major" American Wars of the 18th, 19th and early 20th centure: the American Revolution, War of 1812, Mexican-American War, Civil War, Spanish-American War, World War I and World War II have served in the White House. In fact, the majority of Presidents have been veterans (if you count Reagan, Carter and G.W. Bush).
But it appears that the Vietnam War will join the Korean War as the only major American Wars to not produce a president. The longest span between the end of a war and a veteran of it being nominated for president was the 51 years between the end of World War II and the nomination of Bob Dole. If that is the limit, then Korea's clock ran out in 2004. Vietnam's runs out in 2026. Which gives Vietnam Veterans 3 more chances, but it doesn't appear that there are any who will emerge. After three straight failed nominees - Gore, Kerry and McCain - it's hard to imagine a Vietnam Vet who will run or be nominated. Many of the Vietnam Veterans who've served in congress have retired, and Bob Kerry lost his race to rejoin the Senate.