Vendors keep 'dry ice' in their ice cream boxes in order to prevent melting of dry ice.
Dry ice melts if it is placed in the open space because the carbondioxide gas is compressed with pressure. As carbon dioxide changes to gas, it escape to the atmosphere. If you use an airtight container such as ice cream box, the gas cannot escape.
So we can conclude that vendors keep 'dry ice' in their ice cream boxes in order to prevent melting of dry ice.
Learn more about dry ice here: https://brainly.com/question/14251211