Does the Stomach Vacuum have some opposite exercise to it, like expanding the stomach? Or is this useless?
Stomach Vacuum is contracting the transverse abdominis or TVA. Doing it regularly helps you tighten up those muscles which is both aesthetically desirable and has other health benefits. Aesthetically desirable because it makes your waist look smaller (front-to-back width) and helps you keep your stomach flat and tight while resting/walking. TVA also plays a huge role in preventing back pain.
Now the questions is what would you want to achieve with an "opposite" exercise? If you want more muscular abdominals, you should increase the workout volume on them just like with any other muscle. But this is by no means opposite to doing "Stomach Vacuum" exercise - actually it could only help you achieve better overall definition.
If you've ever heard professional powerlifters talk about how they squat, they often mention the concept of "blowing up" their core. What I mean by this is, right before they descend to the squat position, they will take a deep breath in through their stomach (NOT chest), and keep it fully engaged throughout the rep. I've heard that this is why most lean powerlifters have "blocky" abs.