I have never routinely cleaned decks and in 50 years of mowing never had a deck rust problem. Most deck rust through problems are caused by decks that get a build up of dirt and debris on top that never gets cleaned off. Left outside to get wet in the rain they eventually begin to rust. The underside of decks gets blasted when the mower is used and almost never has a rust problem.

Moisture barriers and vapor barriers are NOT the same thing. Moisture barriers are intended to stop the movement of liquid water. Vapor barriers are intended to slow the movement of water vapor. The bottom of wood floors is not normally finished so a barrier is used to try and slow the annual movement of water vapor in and out of the wood so it more closely matches the movement through the finished top surface. This helps control changes in shape and size of the strips, but cannot stop it completely. Chapter 3, Physical Properties and Moisture Relations of Wood from the Wood Handbook (USDA) is linked below. Figure 3-3, p. 8 shows how all wood changes shape and size as the moisture content varies. Here is a link that might be useful: Wood Handbook, Chapter 3

Yes, I finally had mine done a week ago with metal roof panels. I had read some old messages on here about it last year, and went ahead and finally had it done this year. We used "PBR" style panels that have 1-1/4 inch deep humps. I went with white color so it looks brighter underneath the deck, and don't really notice it from above. We used 2x6's attached to each existing joist on an angle to form a slope, and then used 2x4's across on 2-foot centers to form a frame to mount the metal panels too. We didn't screw through the humps because that's where the water drains down, we screwed right next to the humps. We also installed a gutter on the low side and tied into an existing downspout. Had a few rains so far and it has been drip free! I'm not sure how to add photos here...

You could try putting in a deep root barrier of a couple feet deep the width of the deck, though I am not certain it would work. Trumpet vines are quite vigorous in sending out suckers. You could replace it with another native, trumpet honeysuckle, Lonicera sempervirens, which doesn't sucker.