Its conceivable that they've had a massively larger budget to work with that it could be possible. I haven't been able to play Witcher 3 but it seems like it is possible to do so with out sacraficing dialog options. although the trade off would probably be having any thing in the way of variety in choice of character voice.
I don't think that its actually going to happen either at this point, I find it more likely that they felt it would make for a more dramatic trailer if the vault dweller said something at the very end. but then again it does seem odd to hire a well known voice actor like Troy Baker to just say that one line if that were the case. Then again Troy Baker voice acts in damn near every thing that is game related. There really isn't enough information to do more than wildly speculate.