Involved in the biosynthesis of the antifungal agent validamycin A (PubMed:16725283). Catalyzes the condensation between GDP-valienol and validamine 7-phosphate via a nonglycosidic C-N bond formation to yield validoxylamine A 7'-phosphate (PubMed:21766819). GDP-valienol + validamine 7-phosphate = GDP + H(+) + validoxylamine A 7'-phosphate kcat is 7 min(-1) with GDP-valienol as substrate. kcat is 7 min(-1) with validamine 7-phosphate as substrate. Homodimer. Belongs to the glycosyltransferase 20 family.