Well, they say a tax audit is the mother of figuring things out. Or something like that. We were audited by the state of Washington, and I had to get into my old Seller Central account to prove some income, and — wouldn't you know it — there is a way. It's just in teensy-tiny-font form.
Here are some screen shots, in case you're as near-sighted and unobservant as I am. You can click on any of the pictures to make them larger.
This is the view from within the publisher interface, Amazon Vendor Tools and the Search Inside The Book PDF Submissions area. Up in the toppish left corner is the word "Vendor" (which apparently was supposed to mean something to me) next to a drop-down menu.
To sum up, you can have both a Seller Central / Amazon Marketplace account and an Amazon Vendor Tools / Publisher / Search Inside This Book account. No worries, no problems — I just wish Amazon had made that drop-down menu a little more obvious!