A vegetable stall is a stall, which may be stolen from at level 2 Thieving, granting 10 Thieving experience. To thieve from the stalls in Miscellania and Etceteria, you have to complete the Throne of Miscellania quest. Thieving from the Miscellania stall will lower your approval rating if you are managing this kingdom. Interestingly, stealing from the Etceteria stall doesn't lower your approval rating. Possible loot includes potatoes, tomatoes, cabbage, garlic, and onions.
It is also possible to trade with the Greengrocer at the stall to buy the items instead.