If Vitter brings a defamation suit, then anything relevant to his prostitution-related conduct becomes fair game for the TP's attorney, including depositions.
Even if Vitter is telling the truth about his NOLA activities -- and right now, he may just be betting that Flynt is bluffing -- he may have other yet unpublicized whoremongering or other doings that he would just as soon keep out of a court of law.