In regards to the orientation of rank and branch insignia for officers, the officer in this photo, then 1LT John Gross who was the commander of C Company, 2d Battalion, 47th Infantry (Mechanized), 9th Infantry Division is wearing subdued patches and insignia with the exception of his Ranger tab. This photo was taken late 1967 / early 1968. Note how the 1LT bar is is in line with the lower edge of the collar as is the branch insignia.
This next photo shows Soldiers of track C-13 wearing subdued 9th ID patches and rank insignia on their sleeves as well as one Soldier wearing the heavy cotton fatigues with color patches. This photo was also taken late 1967 or early 1967.
Also at this time, locally made pin on rank insignia was worn on the collar of some Soldiers in the unit. In 1968, US made pin on rank insignia started to be implemented into the supply system, but rank sewn onto the sleeve could still be seen into the 1970s in some units. For enlisted personnel, rank insignia was either sewn onto the sleeve approximately midway between the elbow and shoulder seam on both sleeves or if pin on insignia was worn, it was worn on both collars. For officers, rank was on the right collar and branch insignia was on the left collar of the officer wearing the uniform.
This subject could be more in depth, but I think what I showed and explained should be sufficient for the average modeler new to modeling Vietnam subjects.
Cheers,
James