Time is relative here, and I think the 100 years guideline works fine, but is also a bit arbitrary. The country has changed considerably since 1986, actually both the US and Vietnam have, and we are still reading and discussing it, for going on a quarter century now. I think it will be a very important link for future generations as to what it was like to be in that war specifically, and in war altogether.
Well, the only real test to see if it has stood the test of time would be to ask the same question in about 80 years, 100 years after its publication. It is impossible to say otherwise. However, after the sad experience of the other inevitable wars that humanity is going to be engaged in, if this work still manages to capture universal truths about war, then it will still be read and studied.
It is too early to know if it has stood the test of time. perhaps in a hundred years during other wars, if we are still around then, and people are reading it and saying that it was hard to imagine what fighting was like in Vietnam, then I guess it would have stood the test of time.