Ha. I feel you. Sadly, that vest probably won't help as much as you'd like.
Corey is my guide dog, wears a BIG YELLOW SIGN on her harness that says "DO NOT PET ME I AM WORKING." There are 4 general reactions.
1. The people who just completely ignore Corey like they are supposed to.
2. People who make sounds and faces at her but don't pet her - even though this is just as distracting.
3. People who pet her and then when I tell them not to, they say "Oh, I didn't notice the sign." (This amazes me because it's HUGE and yellow. Legally blind here and even I can see it.)
4. The worst - people who pet her and when I say not to, they say "Oh, I know I shouldn't but I COULDN'T RESIST." As if this is an excuse.
I just don't get it. I mean, it's one thing to pet a strangers dog without asking - it's potentially dangerous for all parties - but to pet a dog CLEARLY marked as a service dog because you "couldn't resist" is just insane.
I hope you do have better luck with a vest. It'll probably help for some people, but sadly, probably not for all.
Corey - female GSD, b. 12/21/10. (Fidelco Guide Dog)