The fish-sauce-based dipping sauce commonly served with Vietnamese spring rolls is called nước chấm.
Nước chấm is a broad Vietnamese term for dipping sauces, and one of its best-known versions is based on fish sauce. A typical preparation combines fish sauce with water, lime juice or vinegar, sugar, garlic, and chili. The proportions vary according to the dish, region, and personal taste, producing a balance of salty, sweet, sour, and spicy flavors.
With gỏi cuốn, or fresh Vietnamese spring rolls, nước chấm is commonly served alongside peanut sauce. It can also accompany grilled meats, rice vermicelli dishes, bánh xèo, and other Vietnamese foods. The sauce is usually lighter and more pourable than straight fish sauce, which is intensely salty and is often called nước mắm when referring to the fermented condiment itself.
Hoisin sauce, sweet chili sauce, and soy sauce may appear as alternative dips, especially outside Vietnam, but they are not the fish-sauce-based Vietnamese condiment identified by this answer. Recipes may be called nước mắm pha when the fish sauce is mixed with other ingredients.