Factors that improve your chances of having a successful vaginal birth include having a normal-sized baby in the vertex presentation (head down, not breech); a normal maternal weight for height; no significant medical issues for the mother (like diabetes); and no medical issues with the baby. However, there is always a risk for failed trial of labor, requiring a repeat cesarean section.
Additionally, there is a one percent risk for uterine rupture if you had a prior low transverse cesarean section. A uterine rupture can lead to catastrophic outcomes for the mother (including hysterectomy and profound blood loss requiring transfusion), and the baby (including permanent brain injury and death). Therefore, during the active period of labor, continuous fetal heart monitoring is done to watch for early signs of fetal distress or uterine rupture.
As with most vaginal births, most women who choose VBAC can safely use pain medicine during labor.