**Yes, the claim is approximately true.**

From a 1976 [study](https://www.tandfonline.com/doi/abs/10.3109/00952997609014295?journalCode=iada20):

>Two stages of Vietnam drug use are identified-a period of increasing marijuana use followed by the 1970 influx of highly potent heroin to which 1/5 of the enlisted troops were addicted at some time during their tour. ... Since 95% of those who were addicted to narcotics in Vietnam have not become readdicted, the situation does not appear to be as severe as originally supposed.

Thus, 20% of US soldiers were addicted to heroin at some point during their tour and 95% of those addicted did not become re-addicted after the war and stopped using heroin.

[This](https://onlinelibrary.wiley.com/doi/abs/10.1111/j.1521-0391.2010.00046.x) 2010 paper more specifically mentions only 5 percent of Vietnam war veterans took heroin 1 year after the war. The authors also write (emphasis added):

>Eighty-five percent of the men told us that they had been offered heroin when they were there—often quite soon after their arrival...Thirty-five percent of Army enlisted men actually tried heroin while in Vietnam, and **19% became addicted to it**.

On 15 May 1971, the [New York Times](https://www.nytimes.com/1971/05/16/archives/gi-heroin-addiction-epidemic-in-vietnam-gi-heroin-addiction-is.html) published an article using the term "heroin addiction epidemic" chronicling the high use of heroin (emphasis added; paragraphs not all continuous).

>**So serious is the problem** considered that Ambassador [Ellsworth Bunker](https://en.wikipedia.org/wiki/Ellsworth_Bunker) and Gen. [Creighton W. Abrams](https://en.wikipedia.org/wiki/Creighton_Abrams), the military commander, recently met with President [Nguyen Van Thieu](https://en.wikipedia.org/wiki/Nguy%E1%BB%85n_V%C4%83n_Thi%E1%BB%87u) on measures to be taken by the Saigon Government, including agreement on a special task force that will now report directly to Mr. Thieu.

>John Ingersoll, the Director of the Bureau of Narcotics and Dangerous Drugs, also conferred with Mr. Thieu and other officials and returned to **Washington**, reportedly **alarmed** at the ease with which heroin circulates and fearful of the danger to American society when the addicted return craving a drug that costs many times more in the United States than it does here.

>The **epidemic** is seen by many here as the Army's last **great tragedy** in Vietnam.

>Some officers working in the drug‐suppression field, however, say that their
estimates [for addiction to heroin] go as high as **25 per cent**, or more than 60,000 enlisted men, most of whom are draftees. They say that some field surveys have reported units with **more than 50 per cent of the men on heroin**.

In the present day, both [CNN](https://www.cnn.com/2015/12/21/health/vietnam-heroin-disrupting-addiction/index.html) and [NPR](https://www.npr.org/sections/health-shots/2012/01/02/144431794/what-vietnam-taught-us-about-breaking-bad-habits) write that 15% of US soldiers in Vietnam were addicted to heroin. From the second source:

>In May of 1971 two congressmen, Robert Steele from Connecticut and Morgan Murphy of Illinois, went to Vietnam for an official visit and returned with some extremely disturbing news: 15 percent of U.S. servicemen in Vietnam, they said, were actively addicted to heroin. 

To conclude:

>Did 20% of US soldiers in Vietnam use "loads of" heroin?

**19% of enlisted US soldiers were addicted to heroin "at some time during their tour."** 20% is approximately accurate.

>Did 95% of the US soldiers who were using "loads of" heroin stop using heroin afterwards?

**By 1 year after the war, 95% of the US soldiers addicted to heroin stopped using heroin.**