When I read the PDF. San Diego State has to be offered at terms mutually agreeable to the parties. Then it's SDSUs right to join. There would only be the vote (presumably, unless Thompson has been empowered to make that call) on the terms being offered.
http://media.idahostatesman.com/smedia/2013/01/02/11/28/S3EZp.So.36.pdf
http://media.idahostatesman.com/smedia/2013/01/02/11/28/S3EZp.So.36.pdf