with a chicom chest rig, putting the mags in the pouches with the flaps tucked behind will definitely assure retention. With true Soviet suspender belt gear, the tri-cell AK mag pouch, this doesn't work, but chicom chest rigs were way more popular than belt gear with the VC and NVA.
For americans, ALICE is pretty slow, but keeping the mags exposed for fast reloads was done by everyone with more modern gear in iraqistan.
Re chicom chest rig: even if that does work, reload times should be 6-7 seconds, no?
Re American pouches. That might be the case for war in dry areas which are fought by throughly mechanized troops. If you dismount your vehicle, patrol in urban areas, then mount again and drive to base, exposed magazines are maybe an option. But I cannot imagine that in the jungle of Vietnam exposed magazines would be a very good idea.
Also, ALICE with its plastic fasteners was adopted only in 1973.
https://en.wikipedia.org/wiki/All-purpose_Lightweight_Individual_Carrying_Equipment