> I don't think there's a name for "buying makers of complementary goods".
That's still vertical integration, you're just looking down the tube from the wrong end.
Suppose a company that sells PCs also makes CPUs. That's obviously vertical integration, right? CPUs are part of the supply chain for PCs. Likewise if they make SSDs.
But CPUs and SSDs are also complementary goods. They're both sold to PC OEMs in order to produce the downstream product, or end customers when they build their own PC or want to upgrade a component or two.
Entering both of the complementary markets is vertical integration because the obvious reason for a company to do it is to integrate the separate goods into a single offering:
> then they could put a little sample of the medicine in each bag.
But then they have a perverse incentive. You get a bag with five servings of chips and one serving of medication -- the first hit is free. And you shouldn't need the drug to begin with.