The biggest difference between Ukraine and Afghanistan or Vietnam is the ability of people to leave. The borders to Poland are open, almost anybody left is going to be hostile or a collaborator or deluded.
"Plan A", this was not. That would have been motorized transport, which was effectively interdicted.
That North Vietnamese logistics was able to smuggle sufficient tonnage to supply even a limited insurgency in the South wasn't inevitable. And probably couldn't have been duplicated by most other military forces.