* The US Census Bureau doesn't track language data as precisely as it tracks other demographic factors. They collect information on the language spoken at home, but it doesn't always capture everyone who speaks Vietnamese. For example, people might report English as their primary language even if they are also fluent in Vietnamese.
* The number of Vietnamese speakers has fluctuated over time. The influx of refugees after the Vietnam War greatly increased the number of Vietnamese speakers, but that number has likely stabilized in recent years.
However, we can look at estimates:
* The 2010 Census reported over 1.5 million people in the US who spoke Vietnamese at home. This number is likely higher now, given the passage of time.
* Various sources estimate that there are around 2 million Vietnamese speakers in the US. This number takes into account the limitations of Census data.
It's important to note that these are just estimates. The actual number of Vietnamese speakers in the US is likely somewhere between 1.5 million and 2 million.