I've seen several references to troops in Vietnam (and probably other
places) sitting on their
helmets

Not a very comfortable seat!

What I saw was helicopter pilots siitting on their flak jackets.

On the rare occasions that they were 1) provided and 2) we felt they were
required, the crewmen sat on them, no exceptions. Once we got into the doorways
and started our approach to the area where we intended to be more "danger" than
"in danger", we slipped them on the proper way. When you are wearing a plastic
helmet, a lot of the allure of a flak vest fades away...

v/r
Gordon
PS, my experiences in this regard are very limited, but I thought I could share
what I saw directly.
====(A+C====
USN SAR

Donate your memories - write a note on the back and send your old photos to a
reputable museum, don't take them with you when you're gone.