How do I correctly install metal roofing over a workshop with a vaulted/exposed ceiling
All metal roofs leak and condensate and therefore need a "moisture barrier". I call this condensation phenomenon, "The Coke bottle effect." When you take a coke bottle out of the refrigerator, it sweats because the inside of the coke bottle is colder than the outside of the bottle sitting in a warm room. So, your metal roofing will be colder on the "outside" compared to the side on the roof sheathing...thus, sweating and condensation. 

The type of moisture barrier depends on the slope of the roof. The lower the slope, the better the moisture barrier should be. 

For "low-slope" applications, like yours, water is more likely to get blown through joints, up under underlayment (building paper), etc. To protect against this, we use a "peel-and-stick" rubberized underlayment membrane. This membrane is 36" wide and has a sticky backing. It is applied directly to the roof sheathing and lapped 2-3" at all edges. The sticky backing will seal the entire roof. (We turn the peel-and-stick membrane over the edges of the structure about 2-3" to eliminate "blow-back" moisture that could get in under the membrane. )

Then, the metal roofing can be installed directly on the peel-and-stick membrane. When you nail the metal down (hopefully with concealed fasteners) the shank of the nail (or screw) will get sealed by the rubberized membrane. 

You didn't ask, but if you insulate the roof, be careful about venting above the insulation. If you don't insulate the roof, be careful to protect the plywood from the dew point or moisture will accumulate under the peel-and-stick underlayment and rot the plywood. 

By the way, if the existing plywood is bad enough that you can step through, then it needs to be replaced (or new plywood can be added over the existing plywood if it's dryrot and not insect infestation.) Dryrot will stop if the moisture is stopped, unlike insect infestation. They seem to just keep coming back.