Actually.....the Title more accurately should read "Last CH-46 Lost in Vietnam".
Probably the last helicopter lost was the Huey hit by a SA-7 Strella missile while flying the ICCS Mission after the end of the War.
https://www.vhpa.org/KIA/incident/73040710KIA.HTM
There was also the loss of the Jolly Greens in Cambodia during the Mayaguez Rescue attempt.
That was a very tragic ordeal for all involved.
https://www.airforcemag.com/article/0709Mayaguez/