I'd talk to your ob first. He has supported you through your vaginal births, which show that your scar is strong. In the UK many hospitals don't treat a VBAC as a VBAC if there has been a VB in between, if that makes sense - like once you've laboured and birthed vaginally they don't worry about your scar so much anymore.
Talk to your ob and discuss your concerns and IF he is not in agreement, THEN you can look around for other options. With twins anything can happen, so things might or might not be favourable by the time you reach term, for this to go ahead.
I too would avoid a cs like the plague, even with twins, so i know where you're coming from (though i'm lucky enough not to have had to have a cs at all). You sound a very wise woman and i'm sure you'll know the difference between cs being an option you don't want and cs being the ONLY option.
Best of luck hun
keep talking and we'll keep cheering you on
Bx