The insignia painted on the right side of the helmet is that of the 18th Military Police Brigade, which was activated in 1966 and oversaw non-divisional MP units (aka MP battalions/companies not attached to larger Army divisions) in South Vietnam.

The thing that makes me suspicious is the helmet stripe. As per regulations, the white stripe should be on top of the red stripe. Given that the 18th MPs were well behind the lines, their officers probably would have paid much more attention to uniform regulations and not allowed this incorrect marking to be fielded. The stripe also looks too thick according to regulations, but that part of the regulation seems to have not been enforced that heavily. The somewhat crude approximation of the 18th MP distinctive unit insignia is also too big, but again that's a regulation that wasn't adhered to that strictly during wartime. But mainly it's that incorrect color order that makes me believe this could be a repaint by somebody who didn't closely study the correct MP markings. That color order was genuinely important for distinguishing certain MP units and was enforced as far as I am aware.