If it is the last night, maybe toward the end of the cruise, they will give you double the vouchers. That's how it worked for us. They tried to sell us on the OBC but we had the free at sea drink package, had prepaid the gratuities, and did not have any expenses charged to the room. It was the last night of the cruise and did not need OBC. So instead they sold us 4 vouchers for $1000 and received 4 "free." What we liked is that they can be transferred to others. This was pre-covid in 2018 and I still have 6 left. Luckily they have extended the use-by date to 2025.