I have flown 4 different 172's hundreds of hours each, on cross country flights. My preferred altitude range is 6,500 to 12,500 feet. I always select both, and have never had any issues in that operation. Ice has caused engine stumble many times, but not fuel selector position. I think that I recall doing the vent mod on 2 of the club planes, even though none of us had such an event while flying.
The newest one was an M model, the oldest was simply a 172, no letter designator of model. It should be noted that Cessna tried under the most conducive conditions to replicate the alleged vapor problem, without success. Further, some of the owners reporting the problem had been on one tank, and cured the problem by changing tanks. Whether on one or both, the Cessna recommendation is simply to change tanks.
The engine and carburetor have no way of being affected by the position of the fuel selector, so there is something serious wrong with that 172. It may be carburetor, mags, plug wiring, or something else Sooner or later, it will recur at an inconvenient time, and moving the fuel selector is not going to change anything, and you will need a place to land without power.
That plane needs a skilled mechanic, unless the owner is simply reporting the Cessna Service Letter exists.
