string GetString(vector<string> vc, string tx, int idx)
{
    int index = 0;
    string ret;
    for (string str : vc)
    {
        if (vc.size() < idx)
        {
            ret.append(str + " ");
        }
        else if(index != idx - 1)
        {
            ret.append(str + " ");
        }
        else
        {
            ret.append(tx + " ");
            ret.append(str + " ");
        }
        index++;
    }
    if (vc.size() < idx)
    {
        ret.append(tx);
    }
    return ret;
}

Введiть слово:
12
Введiть речення:
as as as as as as as
Результат:
as as as as 12 as as as
Введiть слово:
12
Введiть речення:
as as as as
as as as as 12