I'm pretty sure what HK is saying is that in a district with 12k spending per student per year if even 5% of those kids were to go to a charter there would still be 12k available per remaining student, but the special ed population would now be a significantly larger percentage of the whole and therefore each special ed kid would get significantly less funding than they would have previously. The only way to fix this would be to raise taxes.
Also, your stance does not take into account growth of the private school sector and assumes that there will be no new schools. If this is the case then the vouchers would just be used students currently at private school who can obviously to some extent already afford to attend.
Personally I would probably benefit from vouchers. We send our son to a private gifted school we can't really afford and could use all the help we can get. I still am against vouchers. I would never want my kid to have a better education at the expense of another. In fact, the way things are now, the money the district would be spending on my son this year is now extra they can use to educate the other kids. I would also rather return my son to an inadequate public school education than have a penny of taxpayer money go towards education at a religious school. I firmly believe in separation of church and state as an indispensable aspect of our constitution.
