Fixing vent selector vacuum switch ("wild vents")
It took me a lot of searching to find a fix for this, so I'm posting it here in hopes that it might help someone else out.
I have a 1999 RoadTrek 190 Versatile built on a Dodge B350, but this fix can work on any vehicle that uses vacuum to control the HVAC system. It's common in vans like mine for the A/C vents to switch to defrost on their own when the engine is under heavy load (e.g. accelerating, driving uphill, accelerating uphill, etc.).
This happens because the engine doesn't pull as much vacuum when it's working hard, and isn't normally a problem because the system is equipped with a vacuum reservoir and check valves. It works fine unless there's a leak in the vacuum system, which is what happened in my case.
After a bunch of testing, I was able to determine that the vacuum leak was inside the vent selector switch itself. But, of course, you can't just buy a replacement switch--you have to buy the entire control module which will set you back at least $150, assuming you're doing the labor yourself.
Instead of buying a new control module, I decided to fix the seal inside the vent selector vacuum switch. It's pretty easy to pop off the front of the dash, which exposes 3 screws that hold on the HVAC control module. Remove the screws and you can unplug all of the electrical connectors. I also had to remove the doghouse to disconnect the vacuum tubes that go to/from the vent selector switch--there's a handy little vacuum connector under the dash that can be unplugged to remove the entire module.
Once the control module is out, you can see that the vent selector switch is held together with little plastic clips which can be carefully unclipped to pull the switch apart. It can be separated into 3 or 4 sections, but you don't have to take the entire thing apart--in fact, I don't recommend it. Only pull apart the two sections closest to the vacuum hoses (away from the control knob).
Inside the switch, you'll find a disc with a maze-like arrangement of little walls and channels. Those channels are the paths the vacuumed air takes for different switch positions, and may not seal very well.
To fix the seal, you'll need some vacuum grease. I bought a little container of Dow Corning High Vacuum Grease from Amazon for less than $15. It's a 2 oz. container which will last forever since you only need an extremely tiny amount for this fix.
Using a toothpick, coat the top of each little "wall" with grease. For good measure, I also coated around the perimeter of the disc. Then use the toothpick to clean out the little channels between the walls. That's the main fix. I also put a thin coating of grease around the little nipples that connect to the vacuum hoses. Then I just clipped it all back together and reinstalled it in the dash. Vacuum leak is gone, and now my vents stay where I want them when I'm accelerating.
I apologize for the lack of photos; I meant to document the process, but once I got started I totally forgot to actually take any pictures.
Anyway, I hope this is able to help out somebody else who is having this issue.