Am I sinful if I approve an employee's leave, knowing that he will use it to commit sins, such as adultery and drinking alcohol?

There is no harm in signing vacation requests for workers, and you are not responsible for what they do during their vacations, even if you strongly suspect they will commit something unlawful. The sin of their actions is theirs alone. Your signing of the vacation request does not constitute direct assistance to unlawful acts. It is sufficient for you to advise them to refrain from forbidden things. Allah Almighty says: "And every soul earns not [anything] except against itself, and no bearer of burdens will bear the burden of another" and "O you who have believed, عليكم أنفسكم [take care of yourselves]. Those who have erred will not harm you when you have been guided."