I am trying to think of singers in the folk tradition whom I would describe as "virtuosi"

Paul Brady is the first one who springs to mind.

Mick West is another.

The late Isaac Guillory was unquestionably a virtuoso singer, as well as a virtuoso guitar player.

Tony Rice ( in his prime ) fitted the bill.

Roger McGuinn is a virtuoso singer imho.

The Louvin Brothers too.

Emmylou Harris (in her prime)

not an exhaustive list, and there are many other great singers whom I wouldn't describe as virtuosi ...