Before one bit of promo material was released, Disney/J.J. were not shy on letting the world know that the "big three" of the SW franchise were returning to their roles--Hamill at the top of the list. That was the shot heard around the world. With Luke, he's the face of all things Star Wars, so much that much of the pre-release fan & media talk was "what will he do?" / "when will he appear?" and anything else you can imagine. The interest in TFA relied heavily on the promise of Luke & the other two returning. Even after Han was featured in the trailers, the mass curiosity was about Luke. If that's not enough evidence, once you see the film, TFA--from the Luke-centric opening crawl (he's the first thing listed after the episode title) to arguably 50% of the film being about..finding Luke. Every major TFA character mentions Luke.
The point being that the Skywalker story is what SW is about on film, so seeing Vader (a Skywalker himself and tied to the ultimate SW hero's journey) in RO is understandable--Disney needs that all-important connection (bait), fearing that no appearance or reference will turn off a sizable number of the three generations of SW movie fans.