The graph below indicates how many US soldiers were killed in each year of the Vietnam war. Interestingly despite being a fairly long war most of the killing is concentrated on a brief period in the middle of the war 1966-70. This coincides with the period when the most US soldiers were in the country as Johnson increased the military presence under pressure from his advisors.
The total is about 60,000 Americans killed, the Vietnamese in contrast may have lost as many as 2 million people in the war. For more background to this graph see the Visual History of the Vietnam War.