Vector* vector_create(size_t itemSize) {
    Vector* vec = (Vector*)malloc(sizeof(Vector));
    // ...
    return vec;
  }

This just immediately strike me as poor code.

C is not java, you do not have to "new" up every little structure.

  void vector_init(Vector *v, size_t itemSize) {
    v->itemSize = itemSize;
    // ...
  }

Instantly cleaner, more efficient, less memory allocation/handling needed, lets the user control where the Vector is stored.