Green’s Dictionary of Slang

Marvin (the Arvin) n.

[assonance from Army Republic VietNam; later use is historical]

(US, orig. milit.) a generic name for any member of the South Vietnam military.

[US]N. Proffitt Gardens of Stone (1985) 238: The closest and quickest help just happens to be ol’ Marvin the Arvin.
[US]G.R. Clark Words of the Vietnam War 35/2: ARVN [...] Marvin-the-Arvin.
[US](con. 1964–8) J. Ellroy Cold Six Thousand 320: Chaffee called ARVNs ‘Marvin.’ Pete called them ‘Marv.’.