If you plan to move to emacs bookmarks, I see no reasons to use vtable,
it will complicate conversion of eww bookmarks to bookmarks which is
actually trivial.
I'm not sure what you mean. Vtable should keep the presentation separate from the underlying object type, so changing that later shouldn't be much trouble.
ISTM that switching EWW bookmarks into Emacs bookmarks would be a bigger project, because it requires conversion at runtime, likely some UI, documentation, and testing. That's why I suggest not letting that idea stall this patch, because it may or may not even happen, depending on whether anyone feels like doing it anytime soon.