M.R.
Call a board certified child psychiatrist, if that is not the kind of doctor you have contacted that you mentioned in your post. If I am correct, the Vanderbilt is a screening tool, and you want an evaluation that includes something called the Connors scale.
My suggestion is that you find a board certified Child psychiatrist that is not on any insurance plan, and file this yourself on out of network bennefits if you have them, because you will get into see an unaffilitated doctor more quickly if you need it, although you will pay for that privalege. In my experience, a doctor without ties to insurance is going to spend the time that you need when you need it.
Ask that psychatrist to refer you to a nuero psychologist for an full educational evaluation. This probably won't happen until summer, but you need to own your own evaluation and get it ASAP.
Last, initiate an evaluation through his school district. Children whose teachers think they have ADHD and are in danger of failing a grade should already have been referred, but take the bull by the horns and request one, in wriiting, imeadeately. They may not fail a child with a disablity, or suspected disablity, and your teacher has already raised the suspicion.
Check out www.wrigtslaw.com. This site will help you understand your rights and how to advocate for your son. Start with articles about ADHD and the referal and evaluation process, and be sure to read "understanding tests and measurments for the parent and advocate" several times before you get any evaluation feedback from any source.
Get both private and public evals, and never know less than the school about your child. You should own the evaluation that includes his diagnosis.
Read anything by Dr. Mel Lavine or Dr. Russell Barkely. They have great strategies that you can use now and in the future.
Good luck,
M.