In this play, why is the "tragic flaw" seen more as an error of judgement than a vice? And what messages is Shakespeare communicating to his audience by using this style of tragedy to tell the story?
Romeo and Juliet actually share a tragic flaw: they both love too passionately, too intensely, and they fail to exercise caution, patience, or even common sense as a result. This is why it is more difficult to call their flaw a vice, as you say, because loving too intensely is certainly a different, more innocent kind of flaw than pride or ambition, for example. Further, their flaw doesn't really hurt anyone but themselves; it is not like Macbeth's ambition outstripping his morality, causing him to murder as a result, or Oedipus's pride causing him to bring down the gods' wrath on Thebes.
It seems possible that Shakespeare fails to give Romeo and Juliet a true tragic flaw, some real vice for which we might hold them responsible, because he wants his audience to sympathize with them, not blame them. They are relative innocents, the victims of their parents' feud, and it is likely that Shakespeare wants us to blame their parents for the tragedy rather than Romeo and Juliet themselves. After all, he does paint Juliet's mother as cold and distant, and Juliet's father—when she refuses to marry Parris—as cruel and merciless. Even in the prologue, Shakespeare foregrounds the parents' fight much more than he emphasizes the love between Romeo and Juliet. It seems, then, that he wishes to point out the wrongness of the families' violent grudge rather than the too-loving behavior of their children.