What separates the Shinigami from the Hollows? What's different between these two powerful spirits? With the lesser Hollows the answer is obvious. Lesser are creatures primarily dirven by instinct and hunger, while Shinigami are capable of reason and order. Menos, on the other hand, are not so different as they appear. Both are souls of human beings, both possess instinct and reason, and both are capable of tremendous power. When it's boiled down, Menos and Shinigami are two sides of the same coin. A Hollow's power is forcibly awakened during the wait to be taken to Soul Society, while a Shinigami's power is drawn from within by his Zanpakuto, a gift of the Spirit King. It only makes sense then, that under certain circumstances the two would bleed into one another.
Vizard are the result of Shinigami that develop an inner hollow. This hollow represents the bestial instinct within their souls, and is what they would have become if they had not been reached in time after death. This inner Hollow is the source of a Vizard's hollow powers, and the channel for his growth. By awakening the instinct within, Vizard learn from and grow with their inner hollows.
Arrancar on the other hand, were already Menos, possessing instinct and reason. What they bore in too-limited quantities, however, was restraint and finesse. By sealing away their instinct in 'Zanpakuto' of a twisted sort, these hollows are able to become a blend of Hollow and Shinigami, restraining themselves in humanoid form and developing powers not unlike those of a Shinigami.