Q. In dialogue, when a character says “Nam” referring to Vietnam, is an apostrophe necessary? The official name is one word, yet “Viet Nam” is more historical. That would suggest the apostrophe is not needed?
A. The spelling “Viet Nam” reflects how that country’s name is rendered in Vietnamese, which, according to the entry for Vietnam in the CIA’s World Factbook, is “Cong Hoa Xa Hoi Chu Nghia Viet Nam,” which translates to “Socialist Republic of Vietnam.” Vietnamese words are written using the Latin alphabet (usually with diacritical marks, not shown here).
Nam without an apostrophe would be correct then as a shortened form of the Vietnamese spelling of the country’s name. On the other hand, ’Nam would also be correct—as a contraction of “Vietnam.”
So you could choose one form and be consistent—or you could defer to the OED. Although two of its five quoted examples in support of its entry for the word “Nam” (which it labels colloquial) include an initial apostrophe, the headword in the OED is spelled without one.