Handy is an adjective in both sentences, actually. It's just that it's a complement in the first and attributive in the second. Notice "very" modifies "handy" in both. "Very" is an adverb, so it only modifies adjectives, verbs, and other adverbs. The point though is the distinction between complement and attributive -- な makes な-adjectives attributive, whereas だ is its complementary form.