Private religious schools teach based upon a curriculum that advocates a certain religion. The United States government is based upon religious freedom and cannot advocate any religion over another. So they should not be funding any religious schools in any way, these private schools should have to exist by its own means.
If the government can give money to schools without eductation, even if one pays taxes for that education, but will never realize the intended purpose for their children, then it should not be. But this is the case. When one pays taxes, which go to the school system, if they choose to send their children to a private school, they pay out of pocket, thus they pay for their childrens education twice. This is not fair. Vouchers would simply enable people to be free to choose their religion and what their children should be taught.