I had a S&S done about 3 or 4 days before I went into labour, so I don't if it helped or not... but it did hurt quite a lot, probably a bit worse than a pap smear, although I had one of those at my 6 week post-natal checkup and oh boy did it hurt! I was about 1-2cm dilated at the time, had it done on a Wednesday (or maybe it was Thursday) afternoon, went into labour late Saturday afternoon/evening and had DD on Sunday morning.

I hope it gets things moving for you, and all the best for your VBAC!