I read two different memoirs from Vietnam pilots. Mark Garrisons Guts N' Gunships in which the author flew Hueys in Vietnam, he wrote about the AA but I don't have recollection of him mentioning anything about Migs intercepting them. Then I read Jack Broughtons book, Going Downtown where he writes about flying F104 fighters in 'nam and having some close encounters with VC Migs which were located in the far north. So I got curious that was there any Mig/Huey action going on?
I cannot find any evidence of Hueys being shot down specifically: This does not mean it couldn't have happened, but it does seem highly unlikely: North Vietnamese incursions into southern airspace were fairly limited and not conducted for the purpose of anti helicopter operations. With the US military efforts being conducted in the south, the odds of a Huey being above the Demilitarized Zone were pretty low, so I would be very surprised if I could find a source on a Huey shootdown by a MiG.
However, with a bit of digging I can confirm at least 1 US helicopter was indeed downed by a North Vietnamese MiGs during the war: a HH-53 "Jolly Green Giant", shot down over the Laos-Vietnam border on the 28 January 1970 while carrying out a CSAR (combat search and rescue) mission, when it was intercepted by a North Vietnamese MiG-21 as detailed in this amazing source;
I didn't hear Tom Stump's tape but I do have my own copy. I checked my log book and the date is correct.
I was one of the Sandy wingmen, Sandy 04. It was a bad day all around. The SAR was for SeaBird 02 and the shootdown location was in North Vietnam. While Sandy 01 was ascertaining the exact location and condition of survivors he had a SAM fired at him. SA2 as I recall. I think Capt. John Dyer was Sandy 05. Sandy's 5 & 6 were escorting a formation of 4 Jolly's (70, 71, 72 ,& 77). They had arrived from the West Northwest at about 10,000 ft. Jolly 19 (I think) was the primary low helicopter and he decided to refuel early. A C-130, King 03, and the Jolly 19 headed a ways west at about 7,000 ft to conduct the refueling and I went along to provide cover. We were all holding west along the border in Laos, waiting to see how things were going to transpire.
We were receiving several Mig calls but none of us were familiar with the reference point, CRAB, used by the radar guys. After listening to the tape someone reports that the Mig position is some 15 Miles north of us. Anyway, shortly thereafter, people started yelling "Migs!" and Jolly 71 disappeared in smoke. King 03 dropped his tanks and headed west into the weeds and the now disconnected refueling Jolly 19 did the same. Apparently two Migs made one pass through the holding helicopter formation and escaped to the North. Jolly 72 was in loose trail with Jolly 71 when the missile and Mig passed them on the right and hit Jolly 71.
Well, it was chaos after that!! Everyone in the SAR force was headed west at this time to RTB. My notes say that Sandy 01 was unable to contact any SeaBird 02 survivor. My tape is from the cockpit of Jolly 72 whose Aircraft Commander was reported to be LTC Shipman. My notes also show the AC of King 03 was Lt. Casey Quinn, USCG. I can't remember who was piloting Sandy 01, 02, 03, or 06.
The loss of a "Jolly Green" rather than a Huey to a MiG was a much more likely occurrence due to the nature of the work carried out on CSAR missons, which often involved flying deep into North Vietnamese airspace to recover downed pilots, increasing the likelihood of an intercept. However I cannot find any other losses of "Jolly Greens" to MiGs, which makes sense in my opinion. Even a relatively high performance helicopter like the HH-53 flies far lower and at a much slower airspeed, making an intercept extremely challenging (with the difficulty of spotting a camouflaged low flying helicopter from high altitude notwithstanding).
Just to screw with your head, the opposite occured during the Vietnam war too: in 1967 a CIA operated Bell 205 was able to intercept an North Vietnamese Antonov An-2 biplane attacking a radar installation. Source.