Thanks for posting this thread as important. I was having this same trouble where the vent would switch to the defroster when the engine was under load for a few years now. It turns out my problem was self-inflicted.
When I took the plenum off a few years ago, I forgot to hook the vacuum line back up to the vacuum reservoir and couldn't control the vent switching at all. Not knowing the real connection point was on top of the reservoir (I couldn't see it with the cowling on), I saw what looked like an appropriate size fitting to connect it to. Turns out that was just a plastic post that fit my vacuum line fitting. I connected the vacuum line to that and I could control the vents under most conditions.
After reading your post and seeing the pictures you attached, I realized what I did was wrong and decided to check this again. With the cowling removed, it was very obvious where this line plugged in and my problem is solved! Thanks again!