Derived from the idea of a dirty pint, a half spirits/half mixer pint usually given to someone as a birthday "present", a dirty vase is the group equivalent whereby multiple dirty pints can be served to a number of people after a larger amount of spirits have been mixed together in a large vase.
A dirty vase is a sure-fire way to ensure drunkenness on a massive scale. A dirty vase also has the potential to be dangerous, and so should be approached with caution.
Roger: "So Harry, what are we going to get Fred for his birthday?"
Harry: "How about a dirty pint?"
Roger: "Sweet idea, but how are the rest of us going to get drunk?"
Harry: "Well, Roger, maybe we should make it a dirty vase!"