I think this comment is not truly accurate; the vtbl is more than just overriding (it doesn't really always override though), I can probably describe them properly if Jacek doesn't mind such comments though.
(for starters, something like "dispex is our common IDispatchEx implementation for all mshtml objects, and the vtbl allows customizing the behavior depending on the object." would be more accurate; I could also briefly describe builtin props, dynamic props, and custom props)
For "value" I'd say something like "called when the object wants to handle DISPID_VALUE". For "get_dispid" I'd say something like "called when the object wants to return DISPIDs for custom props."
and so on. But let's see what Jacek thinks first.