Hmm I'd say probably hold off inserting for at least a few weeks preferably until 35wks or so. A low dose orally I'd think would be alright though... What gestation were your dds born? If they were overdue or close to 40wks it would be less worrying but if (like me) you tend to go early anyway I wouldnt risk pushing things too soon iykwim. The other thing you could look into if you haven't already would be red raspberry leaf, drinking the tea is much more potent so kinda same thing maybe take tablets for a while
Of course lots of squaring etc to encourage bubby into position and try 'drop' will help put a little extra pressure on cervix. I really hope you are able to get the vbac you want though