Andy,
Since your addon is VBA-based, you can take advantage of the new 'Document.CustomUI' property. Stuff your Ribbon customizations into this property (it's a RibbonX XML string), save the file, then reopen it. Visio 14 will apply the customizations whenever the document is active. Previous versions naturally will ignore this property and still expect you to use the CommandBars or UI Object Model to plug in your UI. You then just need to wrap your commandbar-based code with an app version check as mentioned earlier.
Note: the Document.CustomUI property is very picky. If you get anything wrong in the XML, *none* of your customizations will appear. Also remember that after setting Document.CustomUI, you must save and reopen the file to see the customizations. Visio applies them at doc open time *only*.
Barry