It's all very interesting indeed. I was just as disappointed as you might be to learn that the marking was not NVA as I thought this would make for a great modeling subject... And this was back in the days when the only T-54 was the ancient and crude Tamiya kit I was going to attempt to scare up.
As I wrote, it's been many years and I've had to try to remember some of what he'd said. He did tell me about the famous historical personage of Quang Trung and spoke of the training school. It was a vague memory that the "729" stood for something too that caused me to search on it in conjunction with the "QUANG TRUNG" name.
So it seems that I got mixed up about soldiers from the training school and rather the marking references the counteroffensive name.
Hard to tell if it was actually crewed in combat or if the marking was put on it after taking it and marking it as a trophy.
I have a magazine stored somewhere in the garage that has a photograph of a T-54/T-55/T-59 in or near Saigon near the end of the war that seems unique. The usual NVA national marking of the red disk and yellow star is bisected horizontally and the lower half of disk is sky blue, rendering the marking as a Viet Cong marking. It's the only piece of armor I'd seen as such. I should build that someday, but I just don't know enough about these series of tanks to correctly model whichever version it is. I'd have to do a ton of research.
Also, I have a photo of a Kampuchean M-113 circa 1980 with the .50 caliber replaced with a DShK and "086" and the then-Kampuchean national marking on the side. This consisted of a yellow five-tower Angkor Wat symbol on a red disk, edged in yellow.
Somewhere I also have stashed away a newsphoto of Vietnamese M-113 with the VN national marking. This is depicted in the 1979 Cambodian invasion. I'd love to see photos of the M-48s that were used, but never ran across any.
And again, stunning painting of your model. I actually thought it was the 1/35 Tamiya kit... That's how good it is!