"when I was released from the 91st EVAC hospital"

This was a makeshift hospital in Vietnam where soldiers were taken for life-saving interventions. Staff would attempt to prevent shock, stop bleeding, and prevent infection until the injured soldier could be transferred to a regular military hospital. They would commonly use helicopters to get the victim out of the battle, and then transport them to a ship or military base.

The 91st EVAC was a team that flew the helicopters and were trained to perform emergency medical procedures.