Seems high. When I went through basic in `66 it seemed it was about 1/3 RA, 1/3 Draftees, and 1/3 Reserve/Nat Guard. In addition, it was the politicians that made this war unwinnable. They were so scared of dragging China/Soviet Union into it that they sacrificed human lives for indecision - a shame.
In `66, anyone with half a brain saw how this was a political war and to volunteer was somewhat myopic. There were no defined objectives or plan. VN was an open ended, unfined mess. Don't get me wrong, anyone who volunteered has my deepest respect and admiration.
From STOLEN VALOR by BG Burkett and Glenna Whitley. P.48. "Volunteers accounted for 77% of the combat deaths in Vietnam. That percentage goes up for younger men. Almost 86 percent of the nineteen-year-olds and 97 percent of eighteen-year-olds killed were volunteers."
I have however, seen the figure of 66% (volunteers) used more often than the higher figures. VFW once used 70%. Now they use 66%. Suffice to say it (the volunteer rate) was FAR higher than the rate of conscripted personnel.