Conde Nast's Teen Vogue, seeking to cement its relationship with its youthful readers, is sponsoring a three-day fashion education program in New York in October. Targeted to young aspiring designers, the event will include seminars and lectures conducted by the magazine's fashion editors on such topics as jewelry design and managing a clothing line. There will also be a live concert and fashion show. Teen Vogue, showing the marketing savvy of parent Conde Nast, has named the event FashionistaU.