Heyla spu! Every mama's mileage will vary, but for me a repeat c/s is a lot riskier than a vbac (and so you know my bias, I've had one c/s, one vbac, and no recorded rupture)...
Yes, the scar can open before labor. However, many times the scar opens a little during the pregnancy or birth and there are no symptoms and no problems as a result (these "windows" and "dehiscences" are sometimes discovered when a c/s is done for another reason). The real "monster under the bed" stories come from catastrophic ruptures which are rare but do happen, and the more surgeries you've had on your utereus the higher your risk for rupture.
There are possible warning signs, but unfortunately not all women experiencing rupture have the same signs. My OBs prefered to monitor vbac mamas by doing blood pressure readings on mama and heartone readings (via doppler) on babe every 15 minutes during the pushing phase. They felt that the sharp drop in maternal blood pressure combined with poor heart tones would be the best indicator of problems like rupture. However, other care providers look for heavy bleeding, unusual pain not related to contractions and located in the front, a sudden "bulging" of mama's stomach, having a babe who is close to being born suddenly "pull back up" a significant distance, feeling baby body parts where they really shouldn't be, or general fetal distress (usually through continuous EFM) as signs of possible rupture.
At any facility should a rupture be suspected it's not a question of the staff being "willing to do a cesarean". The surgery wouldn't really be open for discussion at that point...in the case of a true rupture mama and babe are both at risk for death or disability and a c/s is usually done immediately (most often with mama under general since they need to do things FAST).
Regarding pre-birth planning...some care providers will schedule an ultrasound exam of the scar. This exam can tell you if your scar is "think" (less risk of rupture) or "thin" (which doesn't mean you're at higher risk for rupture, there hasn't been enough research done yet). And I did have a plan B for my vbac. After speaking with my care providers and doing my own research we decided we would go with a repeat c/s IF I had a prolonged pushing stage with significant contractions but no progress. Not just if I had slow progress though, and not just a stall of a few hours...we're talking NO progress despite significant contractions for a long period of time, coupled with fetal distress. I did in fact have a very long pushing stage (about 4 hours) but baby's heart was good, my blood pressure was good, and babe was moving sllllllooooooowly but surely. But I had very pro-vbac doctors. It would be a good idea to make a very clear plan to share with your care provider if you have any doubts about them being 110% supportive of you having a natural intervention free birth.
When vbac'ing (in terms of avoiding rupture or complications), the number one thing is confidence. Giving birth naturally is something you CAN do. A vbac is a birth like any other and there are risks, but it's "just" a birth. Next avoid induction or augmentation of your birth. Don't let them pressure you by saying your babe is too big or your placenta is getting old or whatever their story may be. Obviously pay attention to your baby, how they are moving and how you feel physically, but when your body is ready it will be ready. And augmentation and induction increase your risk of rupture (sometimes pushing the risk through the roof). Don't let them break your water and try to avoid epidural or spinal medication that limits your ability to move and feel. Have a doula who has worked with VBAC mamas before or had a successful VBAC herself, a care provider you trust and who is 100% supportive of natural normal vaginal birth for all women, and know what is going on. Ask questions and ask "why"!
There is some amazing information on the VBAC page at http://www.worldserver.com/turk/birthing/index.html
This website has links to summaries of recent (and not so recent) VBAC literature, lists of VBAC questions to ask your care provider, lists of rupture warning signs and general vbac risks, etc. However, the page is maintained by a family that lost a child to a rupture so there is also a page about their personal story. If you are easily disturbed, scared, hormonal, etc (like I was for my entire pregnancy!
) then don't click through to their personal story page.