Fukushima is well over 2000 miles from the central coast of Vietnam. The radiation would have had to pass a large part of the coast of China to get to Vietnam.
Forbe documented the presence of cesium from Fukushima sources in Vermont here:
http://www.forbes.com/sites/jeffmcmahon/2011/04/09/radiation-detected-in-drinking-water-in-13-more-us-cities-cesium-137-in-vermont-milk/#7cb9ea7e591c
The distance between Vermont and Japan is 6,528 mi.