He was always going to come back... If you look at the episode lists he's listed in future episodes... Sadly people just aren't smart enough. It's not back-pedaling it was just promo all along, which worked.
It's weird. I hated that he left. I was heartbroken when they replaced him with Vinny. But, now that Brian's back and Vinny's been around for at least three episodes, I kinda want Vinny back or, at least. have Stewie convince them to get him as the second dog. At least he'll be more receptive than "New Brian"... and I could picture both him and Brian and Stewie having awesome adventures together.
I'm sure they'll bring him back during the Christmas episode. Stewie said he went forward in time to get his Christmas presents early, during the Christmas episode he can just tell himself to not destroy the machine and then Brian is saved.
