Whoever writes the check dictates the rules. Your point number one that creates a "State Approval" process may only start to prevent fraud, but in the end people will object to the lack of standards for the "public money" being spent on the program. And in the end, private schools won't be, and home education will be more difficult than ever. More than a slippery slope argument, this is at the very nature of governmental to ever expand it's control.
First: Take your money to give to someone else.
Second: Promise to give you some of your money back if you surrender just a little liberty.
Third: Get you hooked on getting back a portion of your own money.
Fouth: Threaten to cut off the money unless you surrender even more liberty.
One article (and not really the best of the lot, just the first one I found)
http://www.freedomofeducation.org/content/freedom.html this one's a little better:
http://www.co-freedom.com/2003/03/vouchers.html
In the 1970's the dept of education was created to improve the quality of education in America. Since then, spending on education and schools has increased so that the US spends more per student than any other nation in the world and class sizes have shrunk with each educational study. Schools get bigger and bigger requiring more and more space and $$ to upkeep. Yet the performance of it's charges (the children) do worse and worse. The typical 8th grader of the mid 1700's had a more demanding education that the corresponding 12th grade graduate. And the literacy rate has fallen as well.
http://www.freedomofeducation.org/content/myths
Government directed education is now and has always been a miserable failure. It is time to accept that fact and return the control and the funding,(really the same thing) back to the families, where it belongs.