Involved in the biosynthesis of the azoxy antibiotic valanimycin, which has an antitumor activity. Catalyzes the reduction of FAD/FMN to FADH(2)/FMNH(2) which are subsequently used for the hydroxylation of isobutylamine by the isobutylamine N-hydroxylase VlmH. It can reduce either FAD or flavin mononucleotide (FMN) but prefers FAD. The enzyme has a strong preference for NADPH as acceptor. FADH2 + NADP(+) = FAD + 2 H(+) + NADPH FMNH2 + NADP(+) = FMN + 2 H(+) + NADPH kcat is 62 sec(-1) for NADPH as substrate (with 40 mM FAD). kcat is 51 sec(-1) for FAD as substrate. kcat is 45 sec(-1) for FMN as substrate. kcat is 6.7 sec(-1) for riboflavin as substrate. Homodimer (PubMed:9287340). It can form an isobutylamine N-hydroxylase two component enzyme system formed of a flavin reductase component (VlmR) and a monooxygenase component (VlmH). Belongs to the non-flavoprotein flavin reductase family.