Um...I don't work for Telltale, so maybe it isn't my place to answer, but let me give my thoughts.
The BttF mod for Vice City is nice and all, but Telltale's BttF game wont be that, and it shouldn't be that. Telltale's charm is making challenging (though not challenging in the sadistic Sierra way), funny adventure games that tell a story. The story and humor is what drives people into a Telltale game, and in a sense they're as fun to watch as they are to play (I've had people willingly watch me just play Sam and Max before. One person didn't even realize I was playing a video game). There's a sort of cinematic appeal to them, and presumably that's why Universal trusted two of their biggest properties in their hands.
Like I said in another thread, Telltale finally getting their hands on a big-name license will come with fans of that license unfamiliar with the adventure game genre and disappointed with what happens. I've heard lots of people talking about what they hope the new Back to the Future game will be like, and most people are under the assumption it'll be some kind of action game. (I've even heard one person say it should be a first-person shooter about Doc and Marty going back to kill Hitler. Because when I hear "Back to the Future video game" I think "World War II FPS.") Naturally, the game won't meet those people's expectations, but, well...disappointment is natural. There's not a single video game (or movie, or book, or anything, really) that's universally beloved. Everything is going to have at least one detractor. Telltale has their fans, BttF has its own, and hopefully there'll be a large enough overlap between the two to make the game a success.
As for whether they feel challenged by the Vice City mod? Well, my guess is this: One's an officially-licensed product and the other isn't.