Vietnamese restaurants, sometimes two to a block, are now found all over the metropolitan area. And Washingtonians are becoming familiar with cha gio, pho and shrimp grilled on sugar cane. But in case you are not, the Vietnam Inn is as good a place as any to begin to learn. Its prices are reasonable; the food may not hit the high notes of a few of the best Vietnamese restaurants, but the choice is broad. Most intersting are the assorted apetizers, a generous sampling of five different ones. Also appealing is beef fondue, paper-thin beef slices to cook in broth at the table and fold into rice paper with lettuce, cucmber and mint. Grilled meats, particularly when served over cold noodles with shredded vegetables, are the best dishes. Sauteed dishes tend to be murky. Service is eager but shy, and the basic motel look of the dining room has been modified by little except silk chrysanthemums on the tables.

Open Monday through Friday, 11 a.m. to 3 p.m. and 5 to 10 p.m.; Saturday, 11 a.m. to 10 p.m. MC, V. Reservations only for large groups. Prices: Main dishes at lunch $2.50 to $6.50, at dinner $4.50 to $6.50.