I don't think it would be nearly as difficult as Vietnam. Vietnam had direct military aid from China coming over the land border, whereas the united States had to stretch their logistics halfway around the world.
If the U.S. hadn't used up our missile and artillery supplies in Iran and Ukraine, I think the majority of the Canadian military and government could be taken out all at once.
Obviously attacking a friendly neighbor is a villainous thing to do, but I don't think the Vietnam comparison is apt.