When you are fighting one mob and a second mob is attacking you at the same time (flanking you) they get +2 to their attack rolls. Defensive Awareness II negates that.
However, what really makes VoV great is Defensive Awareness I which you automatically get with Defensive Awareness II. Defensive Awareness I allows you to retain your dexterity modifier to AC when you are flatfooted. Typically you lose dodge & dex AC when flatfooted making you very susceptible to being hit. You will still lose dodge AC when flatfooted but it's better than losing both AC types.