Reduces achiral isoflavones to chiral isoflavanones during the biosynthesis of chiral pterocarpan phytoalexins. The reduction product is a third isomer, which represents the penultimate intermediate in the synthesis of the phytoalexin (-)-medicarpin, the major phytoalexin in Alfalfa. (3R)-vestitone + NADP(+) = 2'-hydroxyformononetin + 2 H(+) + NADPH Phytoalexin biosynthesis; pterocarpan phytoalexin biosynthesis. By fungal elicitor (PubMed:7866024). Induced by the bacterial pathogens Pseudomonas syringae pv pisi and Xanthomonas campestris pv alfalfae (PubMed:8274775). Belongs to the NmrA-type oxidoreductase family. Isoflavone reductase subfamily.