Since we deal with vintage, steel frames in our hobby/addiction, sometimes we run into problems that are easily fixable, if only we knew how. Here I show how I use an 8" adjustable wrench to open or close the hanger.
First, use a caliper to measure the opening from front to rear. On Campy compatible dropouts it should be 10mm.
If it's narrow at the opening, you need to open it up. Just stick the wrench in and tweak. Start with very low force and work your way up, measuring in between.
That's pretty easy, obviously you can use other tools to do this.
But what if the dropout is too "open"? Measure the same way, if it's more than 10mm at the opening, you can close it up with that same 8" adjutable wrench.
Every 8" adjustable wrench I've ever seen has a "hole" at the end of the lever arm so you can hang the wrench on a pegboard. Hook that on the derailleur hanger stop, and you can use the wrench as a lever.
You've got a ton of leverage, be gentle on your tweak.
Are there other tools that do the same thing? I'm sure there are, but an 8" adjustable wrench is about as common a tool as one can find. Oddly, I've found that a 6" or 10" wrench "hole" isn't quite the right size.