It will be undefined behavior.
Looking at the Standard:
N3690 23.3.7.2[vector.cons]
template <class InputIterator>
vector(InputIterator first, InputIterator last,
const Allocator& = Allocator());
9.
Effects: Constructs a vector equal to the range [first,last), using the specified allocator.
It says that the range has to be [first, last), but the standard doesn't mention what happens if that isn't the case. It therefore is undefined behavior.