This is an 8-week, randomized, double blind, parallel group, 3-arm trial to compare 10 mg/day, 20 mg/day and 40 mg/day as starting doses of vilazodone following a switch from generic SSRIs and SNRIs. Vilazodone HCl under the trade name Viibryd™ is approved by the U.S. FDA for the treatment of major depressive disorder in adults. The purpose of this study is to evaluate the efficacy (how well the drug works), safety (the side effects), and tolerability (how well tolerated) of Vilazodone in preventing relapse or recurrence of depression. As vilazodone is not approved by the United States Food and Drug Administration (FDA) to prevent the recurrence of depression, for the purposes of this study it is considered investigational. The word "investigational" means that the study drug is still being tested in research studies and has not been approved for this use by the FDA.