I don't know if George Lucas was meaning to tap into parallelisms with the Vietnam War with the original Star Wars movie.
I mean, the plot follows the Campbellian "Hero with 1000 faces" storyline to a T.
But I also acknowledge that the Vietnam War was a major event that was going on when he was writing it and filming it, so it most likely had influence at the very least, but I believe it can be argued that Lucas was not attempting to insinuate the Empire was the United States at that time.
At the same time, though, I can see a very clear slant in the storyline that an independent society has every right to consider themselves heroes when going up against a larger, seemingly indomitable force that is doing evil while proclaiming themselves to be the good.