Correct me if I'm wrong, but doesn't he need to lock onto a Ki signature in order to instant transmission? If he is outside the normal realm of space and time, he's not going anywhere. And that's assuming that the material used to construct the Vault can even be damaged by his attacks. For all we know, it could be like the Precursors in the Halo series, whose structures would be completely immune to even Goku's most powerful attacks.