Proposed resolution:

Change the synopsis in [vector]:

iterator insert(const_iterator position, const T& x); 
iterator insert(const_iterator position, T&& x);
void     insert(const_iterator position, size_type n, const T& x); 
void     insert(const_iterator position, size_type n, T&& x);

In the synopsis [vector], there is the signature:

void insert(const_iterator position, size_type n, T&& x);

instead of:

iterator insert(const_iterator position, T&& x);

[vector.modifiers] is fine.