Pit Bulls don’t “sense vulnerability.”
They just attack, suddenly, and without the warnings you would expect from almost any other breed.
You never know when they will go off - and these attacks happen all the time.
The whole “cloud” of the breed variations are time-bombs, and as a former trainer who has known many “pit” owners I recommend that if YOU have one in your neighborhood, you help it disappear before it takes another pet or a child from you.
Throw poisoned meat over the fence. That’s what a group of neighbors did when the cops told them they couldn’t do much about the pit bulls who were running loose and terrorizing the neighborhood. The cops said a dog had to bite someone first before they could do anything about it, so the neighbors did something about it.
What is a good defense? Pepper spray? A bat? We do have pit owners in my neighborhood. I can’t just “disappear” them. But I am terrified when I walk by them even in a fenced yard, with or without my dog.