Past headline- "us combat troops sent to vietnam after gulf of tonkin incident" present headline- "president orders troops to afghanistan after 9/11 terrorist attacks" how are the two events described in the headlines above different from one another?

The United States already had some involvement in the war in Vietnam before the Gulf of Tonkin Resolution.  That involvement stepped up to full-scale commitment of many combat troops afterwards and in the years that followed.  The US was not at war in Afghanistan until after the 9-11 attacks occurred.

The Gulf of Tonkin incident involved some alleging of attacks which, history has shown, did not occur in the way the US claimed they had at the time.  The 9-11 attacks were exceedingly real and visceral.

The initial attack and alleged second attack in the Gulf of Tonkin concerned torpedoes fired at US battleships -- so they were military targets.  The 9-11 attacks were perpetrated against civilian targets such as the World Trade Center in New York City.

So some big differences had to do with the nature of events that preceded the sending of US troops.