I know several Vietnames refugees who would disaggree with that statment. Much the same was said of the ROK forces during the Korean war as was said of the ARVN forces during Vietnam. The ARVN, with some help from US air power, fought off a combined arms assault by the NVA, after most US troops had been withdrawn. Then the US Congress cut off their ammunition, spare parts and other logistics support, and the NVA were successfull in their next assault, which was also a combined arms assualt, with tanks, self propelled artillery and so forth.