Vanny is neither a ship designed for at-level crew, nor GB. It's got weak turning, weak armor, low speed (it can run 40knt only with high level engies without armor; with at-level crew, expect 37 knot or so), horrible spread with at-level gunners, pathetic firepower.
Vanny with 120-level crew in good player's hands can make miracles; someone can do 300k damage with Vanny in GB. And in BB4 1v1, Vanny is not bad, considering its range and short hang-time of shells.
But why would anyone want to play Vanny with 120-level crew instead of QV?
Anyway, Vanny is a "fun" ship. But most of the time in GB, the "fun" belongs to your enemies, not you.