struct WhateverItem {
    String memberA {};
    String memberB {};

    void Jsonize(JsonIO& io) {
        io
        ("member A", memberA)
        ("member B", memberB);
    }
};

struct Whatever {
    using WhateverMap = VectorMap<String, WhateverItem>;
    WhateverMap whateverMap;

    void Jsonize(JsonIO& io) {
        io.Var("whatever", whateverMap, 
            [&](IO &io, WhateverMap& m) {
                StringMap(io, m);
            }
         );
    }
};