Yes, there are hundreds of documented cases of either attempted or successful fraggings among US troops during the Vietnam War. By "successful" I mean cases in which the fragger killed his intended target. All of these fraggings occurred in rear areas. We have no records for what happened in the field. Eyewitnesses have, in some cases, reported fraggings in combat, but eyewitness testimony is notoriously unreliable. Also, separating intentional from accidental friendly fire in a combat zone can be extremely difficult for investigators. In almost all of the documented cases, the fraggee was either a NCO or junior grade officer, and the fragger an enlisted man of lower rank.

Sources:

Cortright, David. Soldiers in Revolt: The American Military Today. Haymarket Books, 1975.

Lepre, George. Fragging: Why US Soldiers Assaulted Their Officers In Vietnam. Texas Tech University Press, 2011.

What is fragging?

It wasn't unique to Vietnam either. From my Grandfather's memoirs, who served in the British Army in WWII:

Soon we were off to Royston to a tented camp for the rest of the summer, there to meet the rest of the outfit, clerks, cooks, heavy lorry drivers, 47 all told. The Commanding Officer of this new concept was a Major De Winton, an infantry officer, reputed to be a cousin of the Queen, and was a Regular Officer. I didn’t care much for him, there was this gulf, and I can’t say I didn’t smile when I heard later, when we were in the desert and he had left us to rejoin his regiment, that in an attack on some enemy position, he had been wounded by 5 .303’s in his back. The Germans don’t use .303 ammunition, we do. At the end of the war, in Italy, now a Brigadier, a woman, a school teacher walked up to him while he was inspecting a guard, shot and killed him with a revolver, for political reasons, it must be said.

Corraborating source: http://www.findagrave.com/cgi-bin/fg.cgi?page=gr&GRid=21622858 Although there's plenty to be found on the assassination, it should be noted I found it hard to find corroborating evidence for his wounding beyond the awarding of a DSO in 1944, which would be the right time period. I doubt somewhat that such a story would make it to print given the circumstances at the time. The relationship between the "blue blood" officer corps and the generally working class enlisted men was a source of much tension, especially after WWI ("Lions led by donkeys" and so forth).

The memoir itself: It's up on Amazon somewhere as an ebook but I'll be buggered if I can find it. Here's a dropbox link in the meantime: https://www.dropbox.com/s/spsyldtnl43qplf/Archie%27s%20Memoirs.doc

This is an interesting question, one that I'm not qualified to answer, but I'd add that the more interesting component is whether it was more common in Vietnam than other wars with comparable troop structures.

It plays into what I believe is the common myth of extreme military incompetence (drug abuse, officer murder, civilian murder) during the Vietnam War - arguably perpetuated by contemporary popular culture with a very specific anti-war agenda. Separating the fact from fiction in the US military during the Vietnam War is an interesting project I'd like to hear more about.