North Vietnam would have been laid to waist by B-52s, the USMC would have launched an amphibious assault on North Vietnam that made Normandy look pale in comparison while US Army divisions would have invaded North Vietnam from the South.
So, asides from internal civil unrest and massive protests, you've just slaughtered millions. Now what do you do with the massive hulk of Southeast Asia you've just destroyed?
We may would have killed just as many people or less than the incremental strategy we went with going into Vietnam. Had we mobilized the conflict like we did for WW2 and launched an all-out war against North Vietnam, we would have destroyed the conventional forces in 18 months and followed that with a two-year counter-insurgency campaign. The protests really didn't break out until the Tet offensive. But an overwhelming victory in Vietnam would have put an end to the protests.
After the war, we would have invested billions in a reunified Vietnam, building modern infrastructure, modernizing Vietnamese agriculture and industry, and establishing a network of bases there. Vietnam would become another success story like Japan and Korea.