We had . . . . . . . . very nice meal. . . . . . . . . vegetables were especially good.

The indefinite article 'a' is used before unspecific singular nouns used in a general sense and beginning with a consonant sound. Definite article 'the' is used before specific things. Here, 'meal' is mentioned in a general sense but 'vegetables' are mentioned specifically as those that were part of the meal.