No, there isn't a high level of dioxin in Vietnamese coffee, because dioxin does not accumulate sighificantly in plants. See INTAKE OF DIOXINS AND RELATED COMPOUNDS FROM FOOD IN THE U.S. POPULATION, which shows dramaticly lower levels of dioxin in plant-derived foods compare to meat (including fish), eggs and milk.
PCDDs and PCDFs previously have been detected primarily in lipids of animal origin, and are usually below detection limits in vegetables and fruits
So, while there is some overlap between the coffee growing regions, and the war-time herbicide spraying, as can be seen below, it is unreasonable to expect high levels of dioxin in plant material. The main concern in the food supply in Vietnam is free range chicken meat and eggs, ducks, freshwater fish, snail and beef. See Environmental health risk assessment of dioxin in foods at the two most severe dioxin hot spots in Vietnam
