My first birth ended in a c/s. My second birth was a successful vbac BUT dd2 developed shoulder dystocia and it was a bit "dramatic" there at the end... she was stuck for several minutes and required resuscitation. I was squatting (unmedicated) when she got stuck, the Gaskin (all fours) didn't work, McRoberts didn't work, and eventually my care provider reached in, twisted, and pulled... leaving me with a 4th degree tear. Thankfully dd2 recovered quickly (she has a few neurological quirks that may be due to the oxygen deprivation or may not, we'll never know) and I recovered as well as possible (no fistula, no fecal or gas incontinence, but I do have minor bladder and uterine prolapse as a result, recovery was hell, and the scar tissue and nerve damage still causes some discomfort 4 years later).
DS was born two years after dd2 and his birth was another VBAC. More than the vbac, the chance of another SD (and to be honest, another tear) was what freaked me out. I spent a lot of time processing dd2's birth, researching SD, and talking with care providers/dh/doulas/btdt mamas. I finally figured out my comfort level, switched from a home birth to a midwife supported hospital birth, explained to everyone that while I wanted another VBAC I would prefer a c/s to another sd/tear and wanted to know if my labor started following a wonky pattern (I pushed for nearly 4 hours in many positions with dd2 before she got stuck and I told people that if I wasn't making CLEAR steady progress then we were going to the OR), focused on spinning babies and hypnobabies, drilled my dh and my doula on all the different SD solutions (duck walking, gaskin, etc), bought a copy of the Labor Dystocia Handbook (by Simkin), and tried to relax.
DS had "sticky" shoulders but nothing like a true dystocia, and he was born after only about 45 minutes of pushing. I labored in the tub for a while and then on my side... ds was born while I was side lying actually. I'm expecting my fourth child in early Sept and I'm trying to keep my balance once again. I'll be with the same midwives and hospital and hope for another VBAC... but I'm leaving my birth plan as it was for ds. If things seem to be going "wrong" then I want to know and I want the option to move to the OR (even though I dread another c/s and recovery).
Oh, my posterior asynclitic c/s born DD1 was 9lbs6oz. DD2 was my vbac with sd and tear and she was 9lbs even. DS was a little sticky but otherwise straightforward vbac at 9lbs8oz. Size can be a factor, but it's not a major factor. One good source for info is:
It's a commercial site (they're working on creating a SD predictive tool) but they have a wonderful bibliography of scholarly papers and "easy to read" info on known or suspected causes of SD along with any actual evidence to back up or debunk those theories.