Some kind of underlayment is required or you risk rot in the T-111 from condensation. But probably not an actual vapor barrier like polyethylene sheet. Tyvek isn't a vapor barrier, it blocks air and liquid water penetration but is still permeable to vapor. So it does not trap moisture and I expect that is what TAKerry meant.
I suggest you check with your metal supplier to ask what they recommend for your locale. They likely have guidelines for warranty purposes if nothing else. The details of the existing wall construction may matter, like what kind of insulation or is there already a vapor barrier on inside or outside the insulation or tar paper under the T-111 etc....
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