I guess my first question would be do you really want to force someone who isn't supportive of what you want to be your carer, or are you just in a position where there are very limited options?
I'll be honest I would seriously doubt there'd be many doctors around that would support a twin vbac after 3 caesars..but I don't know your doc so I could be wrong! But he would be very rare indeed.
To give yourself the absolute best shot at a vbac it's so, so important that your careprovider/s are supportive, and completely onside. Otherwise they will just pull the rug at the first opportunity.
As far as what recourse you have if a doctor refuses to care for you...I'm not sure. If you're public with no other options I'd expect that would be different than if you were private and choosing from a number of doctors. Maybe someone else will have some info.
All the best, and good for you!
And congrats on the twins!