How to initialize C++17 vector of pairs with optional element

You are looking for std::nullopt instead of nullptr.

std::vector<std::pair<int, std::optional<bool> > > vec1 =
  { {1, true}, {2,false}, {3,std::nullopt} };

Or simple use default construction:

std::vector<std::pair<int, std::optional<bool>>> vec1 {
    {1, true}, {2,false}, {3,{}}
};