How do you do that [vouchers] without putting government in control of homeschooling?
When the vouchers law is passed, make it unconditional, and make it so that aspect of it could never be changed.
To make it "unconditional" means that there would be no verification as to whether or not any education is actually taking place. Not that I'd object to that amount of latitude, but it really wouldn't be any different from handing families free money to do with as they wished.
and make it so that aspect of it could never be changed.
That would require a constitutional amendment.
(that's of course assuming that the federal government even has any constitutional business getting involved in education in the first place, which it doesn't)