I think most data suggests a subsequent VBAC
has a slightly lower rupture risk than initial VBAC. The data is mixed on initial VBAC after more than 1 cesarean birth - but it seems to increase risk a little to have had more than 1 prior cesarean.
Most ruptures do occur during labor - but they can occur prior to labor, so one should watch for any signs of bleeding or unexplained pain.
Induction or augmentation - especially with prostaglandins - increase the risk of rupture, but unfortunately not having any of those does not guarantee no risk of rupture.
The only IRL rupture I've seen was at 28 weeks, with a prior classical scar that was not known about and a mama who used cocaine the day of her rupture. She'd actually VBAC'd with that scar twice - by arriving to the hospital in late labor with no prenatal care so her scar type was never verified. She arrived to the hospital at 28 weeks bleeding and complaining of pain and was immediately taken to the OR, and her baby survived despite the fact that the uterus had ruptured completely and her abdomen was full of blood.
I had a successful VBAC client this year that scared 10 years off the end of my life. She had a long, slow labor with a posterior baby, and finally decided on an epidural after being stuck at 8 cms for quite a while, thinking we might be heading to repeat cesarean anyway. After the epidural, though, she dilated fully and started pushing. While nearly crowning, she suddenly clutched her abdomen and started yelling "My scar, my scar"
: My OB back up was in house and came immediately, and we agreed it would be faster to forcep the baby than go to surgery. Baby was born straight posterior (which was the reason for her first cesarean) by forceps and perfectly healthy. Turned out there was likely no rupture, as she had no bleeding, no pain after the baby was out, and there were no fetal heart rate abnormalities. Looking back later, the mama thought that what had happened was as the baby moved lower, her epidural was not working as well and she started being able to feel contractions in the front again and that is what she interpreted as scar pain. After labor was over, she felt that the pain at the end was actually the same as the pain earlier before she'd had an epidural. But man, you just can't ignore someone clutching their scar and screaming that it hurts. Except for this one birth, I've never had a rupture with a client IRL, and never gone to repeat cesarean for fear of rupture, so this experience was unique and quite scary.