vector of pair of non-copyable-non-movable-type and emplace_back

This is essentially a follow up to my earlier question Difference between std::unordered_map < K, boost::ptr_deque < T > >'s operator[] (K const &) and emplace

I am trying to implement a associative vector (call it unordered_flat_map) and would like to support operator [key]

Here is the code...

#include <utility>
#include <tuple>
#include <vector>

struct T
{
    T() = default;
    T(T const &) = delete;
    T & operator = (T const &) = delete;
    T(T &&) = delete;
    T & operator = (T &&) = delete;
};

using S = T;

int main()
{
    using value_type = std::pair < uint32_t, S >;
    std::vector < value_type > testuvm;
    value_type p(std::piecewise_construct, std::forward_as_tuple(2), std::forward_as_tuple());
    //  testuvm.emplace_back(std::piecewise_construct, std::forward_as_tuple(2), std::forward_as_tuple());
}

The commented out line fails to compile, which is what unordered_map::operator [] does if the key is not found. Is there a way to make this work? If not, why is this not possible? Is this due to a limitation of vector?