Why do you think the vietnamese nationalists begin by referring to the declaration of independence & the french declaration of the rights of man & the citizen?

The principles on which Vietnam was declaring its independence were contained in those previous American and French documents.

In September of 1945, HoChi Minh issued the Declaration of Independence of the Democratic Republic of Vietnam.  The Vietnamese were declaring their independence from French colonial control -- so it was directly pertinent to quote French principles of The Rights of Man and Citizen over against the French colonialists.  The Vietnamese also were asking America and the Allies from World War II to support the cause of their independence, not only in line with America's Declaration of Independence but also in line with the principles of self-determination for nations that the Allies had declared at the Tehran conference in 1943.

Cwould be the answer