Thanks for your reply. At the recommendation of a Techsoup Stock phone support representative, I called Microsoft Customer Service. I was eventually routed to Presales & Licensing, where I was told that a Vista Business Upgrade license with Software Assurance would entitle me to Windows 7 Enterprise when it's released.
It was my understanding that Software Assurance allows you to upgrade directly with Microsoft - you're not transacting any new business with Techsoup Stock, and so you don't have to wait for new product versions to be available through Techsoup Stock. Once you have your eOpen licenses, you can get the upgrade just as quickly as businesses that paid full price for their licenses. My question was whether or not Windows 7 Enterprise was considered the successor product for Vista Business for the purposes of Software Assurance, and Microsoft has clarified that - with the answer I'd been hoping for!