My initial instinct was that voucher programs are a very bad thing and ought to be killed with fire. However, I'm trying to keep an open mind about these things and I'd have to say that I could easily support a voucher program under a restricted set of circumstances. This is very much a theoretical "work in progress" and subject to reevaluation-a-plenty.
Education is, of course, nothing like a traditional "market" so market forces don't really apply to selecting which school your child should attend. Therefore, any school which accepts vouchers should be a non-profit entity. I think this is pretty uncontroversial. The idea of cutting costs to increase profits is anathema to providing a good education so this should be a non-negotiable requirement.
Some schools will have limited resources and not be able to accept every applicant. To ensure that there is no favoritism in selecting students, this selection process must be random. The only exception which makes sense is that students who attended the school the previous year should be automatically accepted should they desire to return, and siblings should be able to attend school with their brothers and/or sisters. No other exceptions should be considered. A "legacy" system has serious opportunities for abuse and giving preference for proximity would reduce the goal of school choice. Obviously, home school would not be eligible for vouchers.
The most important restriction is that the "voucher" should cover the entire cost of attending the school for the entire year. There must be no additional costs, not for uniforms or transportation or athletics or anything. If a school accepts vouchers, then that voucher absolutely must cover everything. This ensures true school choice regardless of the economic circumstances or special needs of the students.
If all three of these conditions apply, I think I could wholeheartedly get behind the concept of a voucher system. Students would have the most possible choice of schools, which is the stated goal of voucher systems. So long as voucher proponents are willing to go along with these very reasonable suggestions, I'm in. Otherwise, they're just trying to pull money out of public schools to subsidize wealthy families who are already sending their kids to private schools and that, of course, is wholly unacceptable.