Let me put this another way
How many educated people who:
a) Oppose vouchers
b) Have children who do not attend inner city public schools
would still oppose vouchers if they were the only way to get their child out of an inner city public school? How many of them would accept that their child had to be left in that school because the systemic effects of allowing their child to exit that repulsive school would be dreadful?
Respectfully, I believe the answer is "null set".
Opposing school vouchers is, for basically every single person who does so, a completely costless belief. You get the pleasure of "supporting public education"; someone else's kid, whom you will thankfully never meet, loses their future.
Obviously, this is not exactly a unique phenomenon; most people are more sympathetic to policies whose costs they don't bear. But at least most of the libertarian policy wonks I know have endured extended periods without health insurance. Find me the parents who oppose vouchers when it's their own child who has no exit.