I dislike
Van as a nn for
Donovan, just because the "van" syllable would be pronounced differently as part of the full name and as the nn. I'm not crazy about
Van as a nn for
Ivan, for the same reason.
I guess I would like it as a nn for
Vance. Or if you named your kid
Van Buren or something. Or, on a different note,
Van Halen. Lol.
Truly, though, I don't think it needs to be a nn. I think it's sporting as a full name.