To my knowledge the California Code of Civil Procedure makes no specific mention of the filing and service of a reply to a reply to an application opposing any motion, including as here a motion seeking to set aside a default judgment.
However, Code of Civil Procedure § 1005 talks about the timely filing and service "of all papers opposing a motion and all reply papers".
So, for what its worth, if it were me and the defendant does reply and raises new issues or adds additional grounds in support it its motion to vacate and they are of a material nature as might have decisive impact if taken for granted, THEN I would file and serve a written response contesting the reply regardless of procedure protocol. AND if I felt that I needed more time to properly respond, I would ask the court to grant additional time.
Incidentally what are the grounds for the defendant's motion - excusable neglect (Cal. CCP 473(b) or lack of notice in time to defend (Cal. CCP 473.5)? Or something else?
Lastly, I suppose you are aware that the odds are against you as the courts have a tendency to favor such motions.
In most cases, all that is allowed is the motion, your opposition, and perhaps a reply to your opposition. So the chances are that all there is left for you to do now is show up for the hearing on the motion.