PS: I got this to work: 

template TypedefPolicy(T) if (!is(TypedefType!T == T))
{
    TypedefType!T toRepresentation(T t)
    {
        return cast(TypedefType!T)(t);
    }
    
    T fromRepresentation(TypedefType!T v)
    {
        return T(v);
    }
}

unittest
{
    import vibe.data.json;

    static assert(isPolicySerializable!(TypedefPolicy, QuestionId));
    Json serial2 = 42;
    QuestionId qi2 = deserializeWithPolicy!(JsonSerializer, TypedefPolicy, QuestionId)(serial2);
}

However I still cannot figure out how to serialize to QuestionAttempts as in the original post.