The clean, spare white walls and tile floors are like the simple and wonderful seafood houses of the Mediterranean. The menu is simple -- antipasti of seafood salads or steamed bivalves; a few pastas with light, fresh seafood sauces; some grilled fish and a few seafood stews. No meat at all. The plain white tablecloths are decorated only with Italian bottled water and with crusty hunks of bread. And whatever you order will be straightforward and fresh. The first soft-shell crabs, the most impeccable swordfish, a whole red snapper -- these are what make Vincenzo so reliable. Start with an antipasto assortment -- tuna with lemon, varied squid salads, roasted peppers with capers and the like -- then linguine or homemade fettuccine with clams, garlic and white wine, or with crab, or with just tomato and basil. Then try a grilled fish, with just a touch of oil and lemon. There are several seafood stews, but they are so rich and dense that the grilled fish better suit Vincenzo's style. Alongside, a few bits of garlic-scented vegetables, and one of the well-chosen Italian white wines, perhaps the Tokai. For dessert the cart shows some heavily custarded and chocolate-coated profiteroles, maybe a tart, certainly fresh berries with cream or cold zabaglione. Then typically Italian espresso, a not-quite-hot half-cup, as it would come in Italy. And Vincenzo comes as close as any restaurant in this country to what one would expect on the coast of Italy. 1606 20th St. NW. 667-0047. L $8.75-$12.75, D $10.75-$19.75. L daily ex Sat, D daily. Closed Sun. AE, MC, V. Res sugg. Full bar.