A sign at the Islamic Center of Burlington was vandalized late Tuesday night or early Wednesday morning, police and a center official said. Moazzam Baig, a committee member at the center and vice principal of its weekend school, said someone spray painted USA on the sign, at the beginning of the center’s driveway on Lexington Street. Burlington police Captain Tom Duffy said Wednesday that the vandalism was under investigation. Duffy said no threats were made against the center or its members. The sign was vandalized between 9:40 p.m. Tuesday night when evening prayers concluded and 5:30 a.m. when worshippers arrived for early morning prayers.