After the fall of Saigon to the communists of North Vietnam , the crowning achievement of the U.S. Democrat party of the day. There was a lot of equipment and arms left over, much of it US made and supplied to the Army of the Republic of South Vietnam for their for their survival in their fight to hold off the Peoples Army Of North Vietnam, China, the USSR and Fellow Travelers in the US. After the final fall, that left a mountain of war material left over. Most of it ended up piled up in stacks and left to quietly rust to nothing in the tropical jungle climate.
Some however were saved by the victors to be re purposed and modified for their own uses, a very common thing for the Vietnamese people to do. The AK may be in the minds of millions as the signature gun for the Vietnamese communist but even they recognized its virtues and more modern concepts.
The lighter handier and more accurate M16 and M16A1 certainly had an impact on the Communist forces ( no pun intended) but the one that really must have made an impression was the Colt “CAR15” otherwise known officially as the Xm177 mostly used and identified with US Special Operations forces and MACVSOG in particular. With most of the world not using an AK pattern rifle going with the M16 family and now the M4 or M4 like carbine, the Viets knew a good thing when they saw it.
After cannibalizing parts, sourcing other parts from various countries and using non licensed Chinese rip off parts, Vietnam developed its own take on a CAR15. They call this carbine the M18 reportedly.
While it is obvious that the upper and lower receivers are 60s era Colt made originals, the collapsing stock is the current M4 pattern, as is the hand guards. If those new parts are made to original specs or are pure knock offs from a Chinese airsoft company I have no idea.
The M18 is issued to at least some special units and the coast guard. The carbines pictured above appear to have a newer finish and a suppressor, It is curious that the Vietnamese updated the buttstock and HGs but chose to retain the original smooth A1 pistol grip. Many people would agree with their choice over the A2 grip.
You can read a little bit about it here if you can read Vietnamese. It has very, very little technical details about the M18 and is more of a fluff piece with some CNN level knowledge on the AR15 system but I link it for those interested and able to read it .