Involved in the biosynthesis of the antifungal agent validamycin A (PubMed:16632251). Catalyzes the condensation between GDP-valienol and validamine 7-phosphate via a nonglycosidic C-N bond formation to yield validoxylamine A 7'-phosphate (PubMed:22384130). GDP-valienol + validamine 7-phosphate = GDP + H(+) + validoxylamine A 7'-phosphate Homodimer. Cells lacking this gene are unable to produce both validoxylamine A and validamycin A. Belongs to the glycosyltransferase 20 family.