How we cite our quotes: (Paragraph)
Quote #1
To promote the maintenance of international peace and security in southeast Asia […] (1)
The Tonkin Gulf Resolution begins with this line, which means it's pretty important. Congress is stating right away that the goal here is peace and security. Smart way to start, because they don't want anyone to wonder what America's intentions are. By bringing up these noble aims early (and often), the U.S. looks like the helpful protector, rather than some sort of imperialist land-grabber.
Quote #2
Whereas the United States is assisting the peoples of southeast Asia to protest their freedom and has no territorial, military or political ambitions in that area, but desires only that these people should be left in peace to work out their destinies in their own way […] (4)
Here's the obvious reference to imperialism. Members of Congress aren't dummies, they know that the world might be skeptical of America's intentions in Vietnam. By making it very clear what the U.S. is not going after, they are helping their case against imperialism. Congress straight up says what everyone is thinking— "No, we're not after land or political gain."
Now, whether or not that's actually true doesn't matter, because by stating it so plainly and clearly Congress is putting forth noble intentions. (Just to be clear, the U.S. did not take over any territory. How could we, with that whole losing-the-war thing…)
Quote #3
The United States regards as vital to its national interest and to world peace the maintenance of international peace and security in southeast Asia. (6)
Does this sound like the first quote above? It should. If you want someone to believe something, repeat it. (It's science.) By repeating that America's intentions are peace and security, Congress is making it very clear that the U.S. is not after any imperialistic gains. This is kind of like a safety net—if things go bad in Vietnam (what do you know, turns out they did…), America has some defense if they are ever accused of getting involved for the gain of territory.