THE FLEECE ZIP-UP VEST, the capstone of a new corporate uniform, lurks in air-conditioned corporate cubicles across America. It covers the sweating backs of nervous interns ordering supersize coffees at Starbucks . It’s worn by silver-haired executives in the elevator, heading up to their corner suites. It appears in myriad shades of gray and blue, on men of all shapes and sizes who earn all kinds of salaries. It has become as ubiquitous as the take-out salad in humdrum workplaces, and is slowly supplanting the suit and tie as essential office wear.

Typically, the vest is worn over a button-up shirt and paired with chinos and brown dress shoes of any flavor. “The uniform” is how this ensemble has been branded around the office of one 36-year-old working in capital markets in Pittsburgh, a past practitioner of the much-mocked look who asked that his name be withheld. During a recent trip to New York City this month, he observed scores of men wearing gray fleece vests even as temperatures touched the mid-90s.

The trend is so pervasive that an Instagram account with nearly 40,000 followers, The Midtown Uniform (@midtownuniform), has sprung up to savagely document these corporate clones in cities like New York, Toronto and Washington D.C. The anonymous account adds pithy captions to crowdsourced photos, riffing on the omnipresence of this particular outfit. “Money isn’t really yours unless you’re fully vested,” read a caption on a recent post showing two men in matching pink shirts and blue vests. Despite the implicit ridicule, the comment sections on these photos are littered with friends tagging each other and saying things like, “Bro, this is so you.”

Will Crowley, a 25-year-old investment banker who lives in Hoboken, N.J., is well aware of The Midtown Uniform account, yet it hasn’t inspired him to give up the ubiquitous outfit. “There are times where I can’t just wear a shirt. I kind of have to wear a vest,” said Mr. Crowley. He owns six vests, which he calls a “staple of people who work in finance,” mostly from Patagonia or Vineyard Vines. He rotates through the vests during the week, unless he’s meeting with a client, in which case he wears a suit.

The midtown uniform appears to have taken hold post-2008, when many financial firms loosened their once-strict suit-and-tie dress code. The message was: We know your salary is down, but at least you get to dress casual on Friday! “The payouts regressed, so just like every industry that has payment difficulties, they find other ways to satisfy employees and dress is one of the easiest ones,” said a 35-year-old stock trader in New York City, who also asked to be anonymous. He was on the floor during the 2008 recession, and described how the sport coats and wool slacks gave way to vests and cotton chinos in its aftermath.