I carry a knife and a xenon flashlight when I go out training to the playground. Everybody let's their dogs loose, I love dogs, but some are seriously bad raised.
And putting a vest/sweater around your arm isn't going to do jack shit. The pressure a pitbull or Amstaff is able to develop with it's jaw is enough to severly fracture your forearm in a very nasty way. As someone already said, the viciousness of animals cannot be known.
Best thing to do is search for higher ground, stand tall, make alot of noise and kick it with the heel of your foot in the face. Don't place your foot into their mouth though.
And try to control your fear/shaking, they can smell it.