It's quite simple вкусно and вкусный are different parts of speech: & Вкусно is an adverb for delicious. We use the word "как" (ENG: HOW) for adverbs to ask the question & Вкусный is an adjective for delicious. We use the word "какoй/какая/какие" (ENG: HOW) for adjectives to ask the question. As you see in Russian depending on parts of speech we use different question words for HOW. Saying "это было вкусно" you literally answer the question "HOW was it?" (RUS: КАК это было? so they want to hear an adjective as an answer), which is usually asked differently like "Тебе понравилось?" (ENG: Did you like it?) or "как тебе?" or "тебе было вкусно?" - Saying "это был вкусный... (ужин/завтрак/etc)" you answer the question "HOW was the (dinner/breakfast/etc.)?" (RUS КАКИМ был обед/завтрак/etc ? so they want to hear an adjective in your answer). Such question can be asked in some specific situation where your detailed evaluation or comparison of different meals or places are necessary. It's like the question "How are you?" in English culture. Therefore just answer simply with the use of any appropriate adverb (было вкусно/посредственно/отвратительно/изумительно/необыкновенно).