In actual number's its hard to say, since the Army was loathe to report such an event, however it certainly happened, and it was definitely increasing as the war went on.
Fragging was a symptom of the many ways in which the discipline in the US Army began to slowly unravel over the course of the Vietnam War. Other symptoms of this problem were things like phantom patrolling. Where a unit might be ordered to conduct a long patrol and would instead go into the jungle until they were out of sight, sit in one spot for 6 hours, then return, claiming to have traveled their entire patrol route.
Other soldiers and units would simply not follow orders, combat refusals became a problem. Drug use and general disorderliness also increased as the war went on. Officers who ordered units to do unpopular things, such as dangerous jungle patrols, or who tried to bring disciplinary action might risk their lives since unruly soldiers could attack or "frag" them. The perception was that you only fragged officers who were incompetent, i.e. "deserved" it, but this, like any kind of vigilante justice system, is highly debatable.
A lot of the problem can be traced back to bizarre Army personnel policies that rapidly rotated officers through their positions of command on an individual basis. As a result officers and the soldiers under them were not familiar with one another and had not trained together. Individual soldiers were focused primarily on their own personal rotation cycle because at is conclusion they would be allowed to return home regardless of what the unit was doing. This prompted an extreme short term focus in the minds of officers and soldiers, especially after 1970, when the war was drawing down and it seemed pointless to risk oneself for a war already considered over.
By this time good officers began to avoid Vietnam combat service, seeing it as detrimental to their career. As the war dragged on experienced and competent officers and NCOs were cycled home, only to be replaced by younger inexperienced ones. There was a strong perception (often justified) that the officer corps and enlisted corps were at odds. Officers were perceived as being career driven and willing to sacrifice the lives of their soldiers for medals or recognition. Enlisted soldiers, often conscripts, simply wanted to do as little as possible before being allowed to return home. Together all of these factors created a situation where the officer and enlisted soldiers did not trust one another and could not operate effectively together.
As for the lethality its hard to say because accurate statistics are hard to find. But on the whole its seems to have not been very lethal. According to Thomas Ricks' "The Generals" "between 1969 and 1971, there were eight hundred attacks involving hand grenades that, in sum, killed forty five officers and sergeants." This implies that fragging was not very good at killing someone, but often the purpose was simply to put the officer or NCO out of command, in which case wounding was more than sufficient for the job.
Essentially a breakdown in discipline led to enlisted soldiers attacking unpopular officers and NCOs for real or perceived infractions, such as ordering them to undertake a dangerous task or otherwise putting them at risk.
Soldiers would often roll fragmentation bombs under tents of those from whom they received orders or held grievances with. In 1971, about 96 fragging incidents occurred. According to Howard Zinn's A People's History of the United States, 209 fraggings were reported in just the year 1970. In the Americal (23rd Infantry) Division, in 1971, at least one incident of fragging was reported per week.
Fragging, whether it occurred often or not (it is subject to debate), was on the rise later in the war and can be attributed to a plummeting morale among those fighting in the way.
I know this is a broad explanation, but I hope it helped. Anyone, feel free to argue my conclusion.
This comes from Dirty Little Secrets of the Vietnam War by James F. Dunnigan & Albert A Nofi. Two images from the book on the matter.
What were the deceased's next of kin told? That he died on the battlefield? Was there any internal documentation that told the truth?