There are currently 1.11 million F and M students studying in the United States, a 5.5 percent increase since July 2015, according to the latest SEVIS by the Numbers report.
Viet Nam remains in 6th place, behind Canada and ahead of Japan, with 26,209 students at all levels, as of July 2016. (That figure was 29,101 in March 2016. Keep in mind that there is always a decrease after graduation.)
Viet Nam recorded the second highest percentage increase among the major sending countries with a 7.9% YOY (July 2015-July 2016) increase that was exceeded only by India at 28.5%. I expect this trend to continue.