What are the risks associated with vaginal births after c-sections?

Uterine rupture. Uterine rupture is when the scar of a previous c-section opens. It can cause the death of mother and baby if not recognized and acted upon quickly. It occurs in about 1-5/1000 women who have had one previous c-section. The risk is increased if you have had more than one c-section or if your labor is being induced.
For mother and baby. Some of the associated risks include rupture of the previous c-section scar, distress for the baby, bleeding/hemorrhage from the ruptured uterus, possible fetal death, and failed vbac leading to a repeat c-section (which has higher risks than just an elective repeat c-section). Success can be anywhere from 60-80% with vbac, but not everybody is a good candidate to try.