I would assume that a vest used in county would be ready for a dumpster before a Marines duty was complete. They got dirty fast and the material rips easily.
Most "combat" helmets people claim to be authentic field used examples were in reality probably no where near an actual combat zone.
I agree with the flak jacket quote, with rain and humidity the weight of the jacket would eventually rip and deteriorate very quickly.
Thats probably why you see very very few used in theatre jackets. Although Ive seen a nice door gunner flak jacket, being In the chopper ment being out of the environments way. Hence me seeing the door gunner jacket.
With regards to most people claiming there helmet is Theatre used and that most where never near a combat zone thats a very broad statement. That would depend on the helmet and the details that come along with the helmet/cover etc and the research done.
While used in theatre Vietnam helmets are a lot rarer than WW2 helmets used in theatre, I would say this forum has a fantastic array of Theatre used pieces