There were several legitimate ways 1911ís were released from government ownership
The forerunner of the cmp, the DCM sold tons of 1911ís both older and later A1ís in the 50ís and 60ís
In some marksmanship events like camp perry competitors were also allowed to purchase pistols
Up to early WWII officers were allowed to purchase a personal pistol from the government- the embarkation piers in the eastern US even had a sale office to allow officers to buy a pistol prior to shipping out.
There are reports of some cmp sold pistols being older 1911ís with the finger relief scallops added to ďmakeĒ an A1ís frame ( this is rather uncommon and was not a widespread practice)
As long as an item passes inspection criteria in the military it stays in service
In many military units having the most current issue item is not a high priority!
I at one time was assigned to a medical unit. The pistols they had were in excellent to near mint condition never rebuilt as they essentially saw no use.
One other unit I was in had WWII grease guns in inventory still ( also in excellent condition ) in the late 1990ís
When I deployed to Afghanistan some of our units 50 cal MGís were World War Two production with no evidence of ever having been rebuilt
All this is a long winded way of saying sure a very old pistol could have stayed in service up through Vietnam.
I saw one Special forces guy post a picture of a 45 they had in inventory in Afghanistan about the time I was there, and it was a World War One era commercial 1911. How a commercial gun from 1916 or 17 ended up in the inventory of an SF unit in the fight 90+ years later is anybodyís guess