I too prefered that Kylo Ren spoke with an empty helmet. It lets you believe what you want, whether he really saw visions or he is himself delusional. In fact, despite Force Ghosts being officially real in the story, I always thought it would be better if we were certain they aren't really visible to anyone except Luke or the person "seeing" them, as if we, the audience, are simply having a visual representation of a feeling the Force is giving Luke. You kind of get that feeling at the end of ROTJ because Luke is looking at the three ghosts and no one else notices them, but it always bothered me a little that when you're dead you can basically appear at will, seemingly.
It is likely that Kylo Ren had visions of Vader's past when he touched the helmet, since the movie goes strongly in the direction that certain objects can be imbued with emotions or feelings if they belonged to a Force user, like Anakin's/Luke's first saber. If Kylo Ren saw visions of Vader's past with the helmet, it could explain why he has become so fanatical about finishing his crusade against the Jedi, not fully realizing that Vader eventually redeemed himself in that regard (I believe in the novelization Snoke tells him flat out that Vader was weak for breaking at the end, and it would have been better if he'd killed Luke when he could).
