I'm having a repeat c-section due to issues with my scar.
It is incredibly painful. Like super sharp stabbing pains, feels like a cross between having something extremely sharp being poked in there, and my skin being torn in opposite directions. I've also had a small bleed from about 34 weeks, although the pain started much much earlier for me.
My consultant hasn't told me much of why I'm in so much pain. Some days I cannot stand upright, and spend the day doubled over. They've just told me to watch for gushing blood and unbearable pain.
Since my c-section the area of my scar that's sore now has been sore to press on. Adhesions have been mentioned, and possible uterine rupture, although I have no real definite answers other than they'd rather operate again and see what's going on whilst they have me open.
I'd mention tenderness to your midwife, but don't worry too much. I was told your scar can become tender in pregnancy, and it's usually nothing to worry about. Rupture takes place during childbirth more often than not, and a distressed baby is usually the sign rather than pain. With it being vbac, they'll monitor you well anyway.
They offered to let me try vbac as that is what I originally wanted, and then keyhole surgery if the pain continued afterwards to investigate. But I opted for a repeat c-section as my consultant believed that would be what he preferred, and it means they can in estivate and repair right then and there.
So basically, don't worry. Mention it and they'll keep an eye but you're generally ok to vbac even with scar pain.