Vaccaro is primarily a man coverage safety (although he can play in the box and play zone well too).
Harper is an in the box safety.
They are very different types of safeties.
Although Harper was a top in the box type safety, he can't efficiently cover zone or man and that's why some people don't like him and complain about his coverage.
Kenny's ball skills are also better than Harper; he was recruited as a WR before.