A vertical merger is not, as you might suspect, one with an extra-tall stack of paperwork, but an acquisition in which a company buys a supplier or distributor rather than a competitor. Vertical mergers don’t reduce competition the way “horizontal” mergers between competitors do; indeed, it theoretically benefits consumers by reducing costs and streamlining production. Which is why the government has focused its antitrust enforcement on the horizontal variety, and has left vertical unions alone.
Of course, there are people who think the government should block vertical mergers. These critics don’t look at how much power the combined entity will wield over specific markets for goods and services; they view the size of the new firm as a problem even if it has a relatively small market share for any individual product.