As you pointed out, Congress did vote. That vote constituted a declaration of war. You will note that nothing in the Constitution describes the contents, format, etc., of a declaration of war. Consequently, it is up to congress whether or not to actually use the phrase “Declaration of War”.
But it is a necessity, one tragically overlooked in the case of Libya - giving cover to cowardly Democrats who don't want to be on record voting for war - and to cowardly Republicans who don't want to be on record voting for 0bama’s war.