When std::vector<T> runs out of capacity it has to allocate a new block. You have correctly covered the reasons.
IMO it would make sense to augment the allocator interface. Two of us tried to for C++11 and we were unable to gain support for it: [1] [2]
I became convinced that in order to make this work, an additional C-level API would be needed. I failed in gaining support for that as well: [3]