During the time between the passage of the gulf of tonkin resolution and the start of the tet offensive, what happened in the vietnam war?

Quite a lot happened during that time, since the Gulf of Tonkin Resolution was passed in August, 1964 and the Tet Offensive began in January, 1968.

The Soviet Union increased its material support to North Vietnam.  
Starting in March 1965, the United States began a three-year program of bombing raids called Operation Rolling Thunder.
The US continued to escalate its troop levels in Vietnam, up to 500,000 troops in 1967. 
Meanwhile protests against the war in the United States continued to grow.  Those protests would only grow more intense after the Tet Offensive by the North Vietnamese made it clear that the US was nowhere near winning this ongoing war.

The mercantilism option