The 30-year-old has rejoined his first professional side months after leaving Aston Villa.
The 30-year-old has returned to his first professional club nine years after leaving them for Feyenoord, on a deal until the end of the season (via Sky Sports).
Despite having an offer on the table (via Sky Sports), Vlaar left Villa Park when his contract expired at the end of the last campaign.
During his three years with the Midlands outfit, he made 79 Premier League appearances, scoring two goals.
Following his departure, the Netherlands international underwent knee surgery, and will link up with an AZ side that have fallen from last season's third place finish in the Eredivisie, to now sit in the bottom half of the table.
Along with the moves of Christian Benteke and Fabian Delph, Vlaar's departure has arguably impacted Villa's fortunes this season.
The trio's exits have seen the spine ripped out of the side, and have left the Villains desperately fighting to avoid relegation from the Premier League.