No one can answer your question without seeing the motion and without knowing a lot more facts. Generally, though, you would have to respond to the motion; no procedure exists just to dismiss it.
You will need to draft an Opposition to the Motion to Vacate Judgment, prepare it in the proper format, serve it (by mail), and file it with the court on time, all pursuant to the court rules. Depending upon his legal theory, arguments and evidence, you may need to prepare and file a Declaration, setting forth relevant facts. You will also need to prepare a Memorandum of Points and Authorities.
There are several legal theories which your ex might rely, if applicable. Without knowing the details, and reviewing the relevant court papers, I cannot give you any more detailed advise.
Richard Scott Lysle