Quite a few of them never served in Vietnam or never served in combat. This has been documented in "Stolen Valor" and other sources.

According to this source:

When Kerry spoke before the Senate after his Vietnam experience, he not only trashed the military as an institution, but the soldiers who put their lives on the line. Many who have served in the military, and their families as well, perceive Kerry's public disparagement of our troops as unforgivable acts. Furthermore, in his attempts to disparage the Armed Forces engaged in the conflict in Vietnam, he provided false testimony before the Senate, i.e. he is a liar. He and his organization of protesters presented unsubstantiated and false allegations of atrocities. For example:

Elton Mazione, claiming Vietnam Veterans Against the War (VVAW) credentials, Kerry's organization, along with his friends, John Laboon, Eddie Swetz, and Kenneth Van Lesser. They claimed to kill children and remove body parts as part of the notorious Phoenix program. They were neither in Phoenix nor in Vietnam.

Kerry's VVAW leader friend from 1971, Al Hubbard, lied about being an officer, Vietnam veteran, and sustaining war injuries. Michael Harbert, another VVAW crony of Kerry, lied about his Vietnam service.

Frank Dux: He charged many recognizable Vietnam vets with using techniques bordering on war crimes. Dux was a fraud and non-Vietnam veteran.

Yoshia K. Chee claimed we in Vietnam routinely resorted to the most hideous forms of torture, threw people out of helicopters, and decapitated prisoners. He was a phony.

Mike Beamon, an alleged SEAL and Phoenix assassin, was never in the military.

The senator's own VVAW and similar groups relied upon people like: K. Barton Osborn, a Vietnam veteran who testified of atrocities to Congress. He told of prisoners being thrown out of helicopters, a woman starved to death, a prisoner being killed by a 6-inch dowel pushed through his ear. Osborn was not involved in Phoenix, refused to name names, and provided no documentation.

Lieutenants Francis Reitemeyer and Michael J. Cohn. Both sought conscientious objector status because of Phoenix. Reitemeyer testified to being assigned to Phoenix as an adviser and maintained a kill quota of 50 bodies a month. They became famous as My Lai hit the news. Neither served in Vietnam, in Phoenix, or had any firsthand information. Reitemeyer later denied receiving any assassination training.

From this source:

In fact, the entire Winter Soldiers Investigation was a lie. It was inspired by Mark Lane's 1970 book entitled Conversations with Americans, which claimed to recount atrocity stories by Vietnam veterans. This book was panned by James Reston Jr. and Neil Sheehan, not exactly known as supporters of the Vietnam War. Sheehan in particular demonstrated that many of Lane's "eye witnesses" either had never served in Vietnam or had not done so in the capacity they claimed.

Nonetheless, Sen. Mark Hatfield inserted the transcript of the Winter Soldier testimonies into the Congressional Record and asked the Commandant of the Marine Corps to investigate the war crimes allegedly committed by Marines. When the Naval Investigative Service attempted to interview the so-called witnesses, most refused to cooperate, even after assurances that they would not be questioned about atrocities they may have committed personally. Those that did cooperate never provided details of actual crimes to investigators. The NIS also discovered that some of the most grisly testimony was given by fake witnesses who had appropriated the names of real Vietnam veterans. Guenter Lewy tells the entire study in his book, America in Vietnam.

Kerry's 1971 testimony includes every left-wing cliché about Vietnam and the men who served there. It is part of the reason that even today, people who are too young to remember Vietnam are predisposed to believe the worst about the Vietnam War and those who fought it. This predisposition was driven home by the fraudulent "Tailwind" episode some months ago.

VVAW wasn't just a group of anti-war protestors. Some of the more radical members seriously proposed murdering Senators at a meeting Kerry attended. To Kerry's credit, he spoke out against the idea and resigned from the group soon afterwards. Less creditably, he later denied attending the meeting where the idea was discussed until confronted by FBI transcripts.