Does my vigenere cypher algorithm is optimal ?

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C++

Recently, i wrote an algorithm who encrypt an word into another one with the vigenere cypher and i want to know if it's optimal.
Here it's the code

#include <iostream>
#include <string>
#include <vector>

using namespace std;

char encodeur( char premiereLettre, char deuxiemeLettre)
{

    int premiereChiffreLettre;
    int deuxiemeChiffreLettre;

    int lettreFiniChiffre;
    char lettreFini;

    int difference;

    premiereChiffreLettre = int(premiereLettre);
    deuxiemeChiffreLettre = int(deuxiemeLettre);

    lettreFiniChiffre = premiereChiffreLettre  + (deuxiemeChiffreLettre - 97);

    if (lettreFiniChiffre > 122)
    {
        difference = lettreFiniChiffre - 123;
        lettreFiniChiffre = 97 + difference;
    }
    else
    {

    }
    lettreFini = char(lettreFiniChiffre);
    return lettreFini;
}

string encodeurMot(string mot, string motEncodeur)
{
    int taille = mot.size();
    int tailleMotEncodant = motEncodeur.size();

    string motEncoder;

    char lettre;
    char lettreEncodant;
    char lettreEncoder;

    vector<char> tableau(taille);
    for(int i = 0, j = 0; i < taille;j++, i++)
    {
        if (j == tailleMotEncodant)
        {
            j = 0;
        }
        else
        {

        }
        lettre = mot[i];
        lettreEncodant = motEncodeur[j];
        lettreEncoder = encodeur(lettre , lettreEncodant);

        tableau[i] = lettreEncoder;
        motEncoder += tableau[i];
    }
    return motEncoder;
}
int main()
{
    string mot;
    string motEncodeur;
    string motEncoder;

    int onOff(1);
    while (onOff == 1)
    {
    cout << "what's the word you want to encrypt ?" << endl;
    cin >> mot;
    cout << "Thank you, now what's the word do you want to use for encryption ?" << endl;
    cin >> motEncodeur;

    motEncoder = encodeurMot(mot, motEncodeur);

    cout << motEncoder << endl;
    cout << "do you want to encrypt another word ?(0 for no, 1 for yes)" << endl;
    cin >> onOff;
    }
    return 0;
}

sorry for the variables's names, i'm french.

What I have tried:

i have tried to make it as optimal as possible.