The author of the Kick Ass books, which have been turned into two Hollywood films, wrote on Twitter that his fellow Scot is reminiscent of bestselling author Stephen King in that he is "utterly brilliant but ignored by critics because he's popular".
Vettriano, who is currently enjoying a hugely successful exhibition of his work at Glasgow's Kelvingrove art gallery, has been engaged in a long-running battle with the art establishment and some critics who refuse to rate his art highly.
Ten years ago, he said of the art world: "If they've decided you fit what they like, you'll be in; if they've made up their minds otherwise, you never will be. I appear to be in the latter category.
"I have days when I couldn't care less, and other days when I wonder why the gulf exists.
"There's a snob association: when something's too popular it's regarded as a bit trashy.
"But I would rather my paintings sold to ordinary people, rather than being stacked in a store house at the National Gallery."
Now Millar has rallied to Vettriano's defence with a series of tweets.
"The professional jealousy aimed at Jack Vettriano by the art establishment is equal-parts predictable and meaningless," he said.
"Nothing upsets the art world more than their usually deserted exhibitions being sold out every day for months. Jack having last laugh."
A third message put out by Millar to his 51,600 followers read: "Jack Vettriano is a lot like Stephen King in that he's utterly brilliant but ignored by critics because he's popular. It's that simple."
When one follower responded: "Or because the critics are snobs, who dismiss working-class artists who weren't 'professionally trained'?", Millar in turn replied: "There is nothing a critic fears more than a self-taught artist. He immediately renders their expertise pointless."
The debate over Vettriano's talent as an artist, despite his considerable commercial success, was stirred again last week after he revealed that he intends to return to Edinburgh from London.
He said that part of the reason was the number of "Arab and Russian" people who had been buying up property in Knightsbridge, where he lives, which had in his opinion changed the atmosphere of the neighbourhood.
Reaction to him was sharply divided in reader comments on the HeraldScotland website.
Vettriano remains caustically dismissive of critics who dislike his work.
He said last week that one female critic had "loathed" his Kelvingrove exhibition "because you are a wee fat b******".
l A leading think-tank has called for restrictions to be placed on foreign ownership of homes in London in a new report.
Civitas said it believes overseas investment in properties in London ought to be curbed in order to help reduce "rampant" house-price inflation and ease the shortage of houses in the city.