Vietnam has a total area of approximately 331,344 square kilometers (127,933 square miles)

The total area of British Columbia is 944,735 square kilometers (364,764 square miles).

Canada has a total area of approximately 9,984,670 square kilometres (3,855,100 square miles)

Stretching your logistics is an entirely different challenge than spreading your logistics out.

The USA built up many bases in South Vietnam to launch the invasion. So it's not like they were operating on a major handicap.