Is my work in purchasing alcoholic beverages for company parties, then reaching an agreement with the hotel to give guests vouchers to exchange for whatever they wish, considered a sin? And what is the correct action in this situation?

Arranging coupons that are exchangeable for alcoholic beverages is forbidden if you are certain or have a strong suspicion that those present will exchange them for alcohol, because assisting in sin is prohibited. The Prophet, peace and blessings be upon him, cursed everyone who assisted in [the production or consumption of] alcohol. However, if you are unsure about the attendees' intentions, then that action is disliked for you.