Arrancar are powerful hollows that were altered to allow them to suppress their true powers into a sword. that allows them to release it like a Shinigami releases its Zanpakutō. A Vizard was originally a Shinigami that gained the powers of a hollow (ability to fire a Cero and increased abilities) making them much more powerful. Arrancar never gained any other abilities only to suppress their true power, they also regain some humanity so are no longer driven by instinct but Vizard are basically Shinigami with "extra."