Depends on the period flak jackets were more common in the later year, by 68 the USMC was quite strict on them, especially when operating under PAVN artillery range. Plenty of pictures taken during Dewey Canyon of USMC in flak jackets while in operation for example.
some unit commanders were picky about them other not so much. Keith Nolan, in Dak To, notes that troops from the 173rd ABn Brigade when moving from III CTZ to II CTZ did not use them, suprising severl officers from the 4th Infantry Division. At least one officer from the Ivy Division warned them on the subject (Nolan points out the reason was the scarcity of contacts in the III CTZ after Junction City opposed to the situaiton n the II CTZ and the Central Highland).
Best,
Arrigo