I question that unless you have some facts to back it up. I can’t refute it, either, but I doubt that the “US” or “RA” prefixes made any difference to the computer (yes, they had them back then) that sorted names for assignment.
Inevitable, no. Highy likely, yes. It largely depended on your MOS (Military Operational Specialty, or job description). If a particular MOS needed in Vietnam was yours, you were more likely to go.
The rules back then allowed you to enlist with a guaranteed MOS or training for one if you qualified, and unless you screwed up, that wouldn’t change. They allowed you to specify where you wanted to go in advance of enlistment, and as long as the position was open, you would be guaranteed assignment there, but only for one year. After that, you went where the military needed you most (but you could always request a transfer).
The trade-off often revolved around the enlistment or draft period. Draftees had to serve 2 years, Army enlistees, 3; I believe Navy, Air Force and Coast Guard were 6. If you wanted to be an officer, the minimums were higher in some cases, but I forget the numbers.
Draftees had absolutely no guarantee as to where they would go or what MOS they would have, although they could make a request. Enlistees had some control, but not total. You had to decide what was best for you – taking your chances or not.
And sometimes the recruiters would lead you astray to get points. They didn’t get points for draftees, but they did if they signed you up for enlistment. So they didn’t tell you that, in the first week of basic, the call might go out for qualified parties, and if you responded, you could get the same service as an enlistee did, but for a shorter term.
Example. I enlisted to avoid the draft, was qualified for and guaranteed Army Band for 3 yrs. But in Basic, at one formation, everyone who played an instrument was sent to the band barracks for auditions, and those that passed were send to band assignments right out of Basic. So if I had been drafted, I probably would have ended up the same place, but with a shorter service period.
Band training was 6 months in Little Creek, VA. I know draftees who were sent there, then stateside, then Vietnam. By the time they arrived in Vietnam, they had only 8 months left to serve.