Where is the Tab Order Assignment dialog in Visual Studio 2012?

It is still available, you just need to add it back to the View menu. Tools + Customize, Commands tab, Menu bar = View. Select the menu item in Controls where you want to insert it, say the bottom one. Then Add Command, Category = View, Commands = Tab Order.

Your memory of this command does sound a bit hazy, it was never on the Format menu and its never been a dialog. If you want to put it on the Format menu then you can, just pick the Format menu bar in the Commands tab. You can't make it a dialog though, it numbers the controls in the designer view. Some odds that you actually remembered the View + Document Outline command.

Go to your alignment toolbar. At the end, click the tiny little "Add or Remove Buttons" down arrow. Click the "Add or Remove Buttons" button, and look towards the bottom of the list. You should see Tab Order. Click on that and it will be added to the alignment toolbar.