What should I do after repenting, knowing that I used to buy cigarettes on credit from a vendor who passed away, and I was unable to pay off my debt?

The general rule is that it is impermissible to offer compensation for forbidden things. However, in the case of cigarettes, because there is disagreement among scholars regarding their permissibility, and because not paying the price might give a negative impression of the repentant person, it is better to pay the price of the cigarettes acquired on credit, intending the payment as charity. If you are secure from negative assumptions and demands for payment, then giving the amount as charity is permissible.