Education is not a simple economic equation that can be solved by privatization. There are public schools in Kansas that I'd put in head to head competition with almost any private school in the country. They are successful because of parental involvement and a belief by the administration that every child needs to feel like they are part of a community.
The other problem with vouchers is that it will cause a across-the-board tuition hike because the demand for private education will outweigh the supply.
Statistics that show that private schools provide better outcomes fail to take into consideration 4 factors:
1) they can deny service to any child that doesn't meet their academic criteria, and thus do not have as many unmotivated students pulling averages down
2) they do not have to provide special education services, and thus do not have special education students pulling averages down
3) they can kick out disruptive students at any time, without having to go through 5 layers of red tape
4) they often do not have to take the state assessments everyone else does, thus are not measured by the same criteria