I'm interested in this too - my VBAC failed to progress (waters broke but labour didn't get going so I was induced but not very successfully). We are also considering TTC. Both my sections were pretty straightforward and I assumed I would be strongly advised to have another. My understanding is that the risk of rupture is something like 0.5%?? but the consequences can be very serious as they don't always realise that it has happened until it's too late.
A multivariable analysis of the NICHD study showed that there was no significant difference in the rates of uterine rupture in VBAC with two or more previous caesarean births (9/975, 92/10,000) compared with a single previous caesarean birth (115/16,915, 68/10,000). 46 However, the rates of hysterectomy (60/10,000 compared with 20/10,000) and transfusion (3.2% compared with 1.6%) were increased in the former group. 46 These findings concur with other observational studies, which, overall, have shown similar rates of VBAC success with two previous caesarean births (VBAC success rates of 6275%) and single prior caesarean birth. 39,4749 Therefore, provided that the woman has been fully informed by a consultant obstetrician of these increased risks and a comprehensive individualised risk analysis has been undertaken of the indication for and the nature of the previous caesarean sections, then planned VBAC may be supported in women with two previous low transverse caesarean births.
this is from here - http://www.rcog.org.uk/files/rcog-corp/GTG4511022011.pdf - if u can wade through it, it is very useful!