Green’s Dictionary of Slang

(US) an active protester against the US involvement in the Vietnam War (1964–75).

1966		Maclean’s (Toronto) 15 Oct. 64: I would rather trust the decisions of the elected rulers of 200 million Americans and 10 million Australians than the noisy utterances of irresponsible Vietniks.
1967		E. Shepard Doom Pussy 74: I’d like to get a bead on one of those Vietniks [...] carrying the Charlie flag.
1989		H. Rawson Dict. of Invective (1991) 273: The ending often has a derogatory connotation, as in nudnik, and is particularly useful for stigmatizing those who are going against society’s mainstream, e.g. [...] Vietnik.