Reduces achiral isoflavones to chiral isoflavanones during the biosynthesis of chiral pterocarpan phytoalexins. The reduction product (sophrol) is a third isomer, which represents the penultimate intermediate in the synthesis of the phytoalexin (+)-pisatin, the major phytoalexin in pea. (3R)-vestitone + NADP(+) = 2'-hydroxyformononetin + 2 H(+) + NADPH Phytoalexin biosynthesis; pterocarpan phytoalexin biosynthesis. Belongs to the NmrA-type oxidoreductase family. Isoflavone reductase subfamily.