Fashion editors all over town have gotten the runaround from Lanvin about its US flagship store opening. “They’ve been cagey about giving information,” said one. But the French design house is set to open its doors at 815 Madison Ave. next week. The 6,000-square-foot, five-floor emporium will have 30-foot ceilings, a bridal floor, antique chandeliers and “acres” of mirrors. The space was constructed by Michilli Inc., which has outfitted shops for Vera Wang, Dolce & Gabbana and Roberto Cavalli.