On the morning of Sept. 16, 1994, Xuyen Thi Vu suffered an economic disaster: a D.C. police officer ordered her vending cart towed away from a lucrative spot near the Vietnam Veterans Memorial, where authorities estimated she could ring up more than $1,000 a day in food sales.

Within hours, a rival vendor compounded Vu's sense of injury, parking his hot dog cart in the empty spot at 21st Street and Constitution Avenue NW.