Vietnam was unable to be defeated due to the fact that they were capable of using the terrain unbelievably well. On top of that, Americans were not prepared for combat in that type of situation, nor were they prepared for the morale loss that would occur.
And of course there was the "Make love not war" gathering throng back in America, that undermined the very government itself in it's actions, forcing politicians to think about going against the war in order to gain votes.
I also believe that at one point America was kicking butt in Vietnam...

Now what will happen if there is a rebellion in America?
Well, let's say that we have "Assault weapons". Hell, we'll expand assault weapons to things like assault rifles(Which would make sense..yeah?).
Now, let's be nice and even limit it to a place like the rural area of my hometown, making it so the entire town is against the country. We'll move in an equal number of soldiers, including an equal number of firepower.

Who will win? The soldiers. They have the training, they have the supplies, they have the assistance from outside the area such as information. Soldiers are trained, ordinary people are not. So what will happen if everyone in my hometown has an Assault Rifle? Well...some will shoot themselves, others will shoot friends and a few lucky buggers will actually do good until they get slaughtered by the soldiers.

Now let's make it realistic, in that the soldiers would come with far better equipment and reinforcements. Do you really think the ordinary o' people have a chance? No, they don't.