A method for manufacturing a precursor for a primary preform for optical fibers by an internal plasma deposition process including the steps of providing a hollow substrate tube, creating a first plasma reaction zone having first reaction conditions and depositing non-vitrified silica layers along at least a portion of the inner surface of the substrate tube, subsequently creating a second plasma reaction zone having second reaction conditions different from the first reaction conditions and depositing vitrified silica layers along at least a portion of the substrate tube, and cooling the substrate tube to produce the precursor for a primary preform.