Though on a curious note, if Vivec knows of Baar Dau's fate, why doesn't he choose to stop it, as in truly destroy it? Rather, he just freezes it, only delaying the impending impact. It seems an odd choice to me. Does he keep it around simply as a trophy, a marker of his past deeds, as a form of divine bribery (love me or perish), or for some unknown reason. It seems he must have had a reason, since he seems capable enough to destroy such an object.
As for the things Vivec supposedly did, it does tend to be easier to view them from a metaphorical point. Their purpose is to teach the people lessons?hence the names of the shrines, such as Humility, Daring, Grace, et cetera. The focus isn't on what he did, but how what he did shows how we should be. But still, that's avoiding the question, isn't it?
Simply put, we don't know. My own assumption is that while some events may have an element of truth, the majority of them are hidden under the layers of Vivec's poetry to the point where it's hard to say where he means what he says. He could speak of literal actions, of spiritual actions, of poetic actions, or even of all three at once. The most you can go on is your own intuition, as there is no definitive answer that I'm aware of.
Edit: This forum, so busy during the day?
Vivec does not hold the rock up. The entire point is that he used the power inherent in love and worship to hold to stop it, and faith keeps it aloft. The Dunmer hold it up. The idea of using it as a prison was rather nasty and not of any theological significance. Sounds like Berel Sala's work.
That does change things a bit. Even so, with the powers of a god and chim, it seems he would still be capable of eliminating the threat, yet chooses not to. It seems hard to imagine that he is (or at least was) incapable of doing so?
Edit 2: Quick Edit, I hate you and your apostrophe-corrupting powers...