Maguire moved to Old Trafford for £80 million (A$145 million) in 2019 but the England defender is yet to exceed van der Vaart's expectations.
The former Tottenham Hotspur star cut a stricken figure following United's 5-2 victory over Bournemouth, during which Maguire was nutmegged en route to Junior Stanislas' goal.
"I think Maguire goes home every day and tells his wife: 'I'm sh*t at football, but I earn so much. They really believe I'm good'," van der Vaart told Dutch TV channel Ziggo on Sunday.
"I really think so, he laughs at everyone."
The Dutchman's comments are the latest in a series of scathing assessments, with the other coming after United's 1-1 draw with Liverpool in October.
“If he is worth so much money, Virgil van Dijk is worth €300 million (A$487 million)," van der Vaart said.
“If I am going to play with the amateurs on Sunday afternoon, I can easily find three who can do this (what Maguire does on the pitch).
"I mean that seriously. A bit silly to say, but I really think so.”
Despite the criticism, Maguire returned to the pitch on Friday as United ran out 3-0 winners over Aston Villa.
