As the US got more heavily involved in Vietnam, the need for unit patches had to scale rapidly. Often these were supplemented by individual units who would have them made locally based on their own drawing or design. The patch would be ready next day and the design kept as a template for future orders.
Michael Burroughs of Anything Militaria says that the key to collecting Vietnam patches is to look out for when and where they were made. Beyond that, it’s worth keeping an eye out for any unusual patches, such as might have been made for small special forces units.
As an example, a pair of hand made badges like these would be highly sought after. They were commissioned by the Green Beret Special Forces who made up the first boots on the ground in Vietnam. Their official designation here was the Military Assistance Command, Vietnam, – Studies and Observation Group (MACV-SOG).