Thinking about it further, I would expect any water seepage through mortar and brick to be slow enough so it wouldn’t be a problem. The slight gap between bottom flashing and counter flashing should shed the water vapor fast enough.
Also, shouldn’t any vapor barrier be placed on the warm side?.. meaning that if this problem was true, the inside of the chimney would need the tar.
My buddy is not a roofer and he was talking in general terms. We were discussing my roof and all the fine points of my DIY job.
On my chimneys, I removed the 46-year-old flashing and ran I&WS up the sides from the roof, then reflashed (using good roofing techniques) with galvanized steel. I don’t like aluminum because of excessive thermal cycling associated with it. I cut into the mortar and filled the cut with roofing tar before setting the top lip of the counterflashing.
gtp, I’m not a roofer, but for some reason roofing fascinates me as a hobby. I have a lot of respect for the experience people have on this board.