So I've decided to explore outside the bounds, and I received a marvelous surprise when Vardenfell was solid!
Unlike the fake one on the main Skyrim map which is unsolid.
It appears cut off, as if it is missing a few pieces, but the majority of the island is there, and in a solid form.

You can even walk up to the Red mountain and scale it and so on.
It was definitely fun to explore it. Maybe a modder can one day populate it.
Oh and...The Mountain is much further away than what it looks like from Solstheim.

Here are some screenshots showing it.
(No idea why links are disabled, I hope you can manage to see the screenshots either way by copying the screenshot into your address bar and removing the spaces between c and om, and the one between p and ng, and replace the xx with tt.)

hxxp://i47.tinypic.c om/2eukaie.p ng

hxxp://i50.tinypic.c om/2a00pig.p ng

hxxp://i49.tinypic.c om/2jev42a.p ng