Based on the extremely negative response to Family Guy losing Brian Griffin, I do not think the Family Guy cast will allow it to stay this way for long. Writers could create a plot twist, such as Vinny turning out to have dark intentions, and Stewie finally being able to rebuild his time machine due to a slight plot change. Also, future episodes listed online of Family Guy have mentioned Brain.
Of course Brian Griffin will return to Family Guy. The "killed" him off to give the show a little bit of drama, but there is no way that it is permanent. The chemistry and relationship what Brian and Stewie had is very important to show and they will lose viewers if they do no bring him back.
The is only the beginning of a story arc that well end soon there is 1 support of evidence, the episodes with Brian's names on them trust me this is a story that will end before epipisodes with Brian's name on it. Weather you like it or not Brian will come back.
I'm pretty sure it was just an attempt to stir up controversy and bring in more viewers. This season will feature episodes called "Brian's a Bad Father" and "Brian the Closer". My prediction is he'll be back in another later episode known as "3 Acts of God" or the Christmas episode where Stewie will "devise a master plan to get the one and only thing he wants for Christmas".