Ouch, I'm afraid I don't know the aneswer to that one...might take me a few days to get back to you. I know for sure that at least one C-130 was engaged by a MiG during the war, because one of my tech school instructors was the crew chief. They managed to escape the MiG, but there weren't any canyons involved, just a lot of jinking and some friendly fighters.
Please let me know if you come up with anything.
It's probably a story concocted by a C-130 pilot to pull someone's leg.
I googled "C-130 MIG Vietnam canyon" (shoulda thought of that before) and got the story from:
http://uscgaviationhistory.aoptero.org/history06.html
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A report came in on a MiG in our vicinity. This is when the first MiG hit the formation. The first missile missed and the second hit Jolly 71 and it disintegrated. Everyone started yelling --- Migs - Migs --- TAKE IT DOWN! --- We headed for the weeds." The Helicopters made for the ground at max rate and Casey sent his C-130 diving toward earth. The refueling baskets for the helicopters were larger than those used to fuel the jets and had a max speed restriction when extended. This speed was exceeded before the drogues were fully retracted and both were lost on the way down.
A second MiG-21 joined in and came after Casey's C-130. Casey by now was at tree top level. Jolly 72 called out that a MiG passed his right side and was headed for Casey. When Casey was back at Eglin AFB checking out in the HC-130P he had stated that the "Herk" performed so well that it was like a four engine fighter plane. He was going to get the chance to prove it! Casey said he knew it would take three to five seconds for the MiG to get a missile lock, so he picked the canyon just ahead and jinked and flew his "Herk" in a series of unpredictable erratic maneuvers between the walls at tree top level. No missiles were launched that Casey knew of but he could see the bursts from the MiG's cannon hit the karst ahead. He said he was too busy to dwell on it. Moments later the C-130 emerged from the canyon --- the MiG-21 never did! Casey got his MiG, but due to the chaos that existed, never got confirmation on the "kill."