A few days ago, I saw two or three friends from Seattle years posted on Facebook a tribute on Raymond Hunthausen, former archbishop of Seattle, on his death at 96 years old. (The writer is Fr. Michael Ryan, rector of the Cathedral parish and one of the spiritual chaplains of the L’Arche community that I belonged.) By a coincidence, the next morning I came across two items while looking at some old issues of a magazine by Vietnamese Catholic refugees, and one of them shows a photo of the late archbishop presiding over a mass among Vietnamese refugees in 1978.
The short answer: There were about 900 Catholic priests of Vietnamese origin in the U.S. by 2012. There are approximately 950 at this time (2017), and probably more.
The article is accessible at this link, but you’ll need a subscription to Project Muse or institutional access to ACS.