It's not really clear whether Ren is communing with Vader's living spirit, but what's clear from the novelisations is that as an artifact of evil, imbued with dark Force, the helmet remains a potent object through which one can access dark Force visions in a similar way to Rey experienced Force visions when she touched Luke's lightsaber.
From the Junior Novel
He knelt in his dark chambers aboard the Star Destroyer. Though he was
alone, he spoke in a low and respectful tone. “Forgive me. I feel it
again. The pull to the light. The Supreme Leader senses it. Show me
again the power of the darkness, and I will let nothing stand in our
way. Show me, Grandfather, and I will finish what you started.”
The shrine’s centerpiece was a symbol that once was feared throughout
the galaxy. Flames had warped and melted it into a shape even more
monstrous than before. Though cool to the touch, evil emanated from it
as if it continued to burn.
The Official Novelisation has a slightly different take, emphasising that whatever answers Kylo Ren has been getting, they're probably coming from his own fevered imagination, not from Vader.
The alcove where Kylo Ren was kneeling and speaking was darker than
the rest of the adjoining chambers. He kept it deliberately so, as
seemed appropriate for its function. He spoke now in a tone different
from the one he usually employed when conversing with others. There
were no orders to be issued here, no pathetic underlings to command.
The one with whom he was presently communing would understand
everything Ren chose to say, in whatever voice he chose to employ. No
need here and now for intimidation, for fear. Kylo Ren spoke, and the
object of his words listened in silence.
Trembling slightly, he rose from where he had been kneeling and strode
off to another portion of his private quarters. There was no response
from the one to whom he had been talking: neither argument nor
agreement. Only silence from the shape that had been the object of
Ren’s fervor: a ghostly, deformed mask that had once belonged to
another. To a figure of rumor and legend and fear.