Vader will likely have an appearance at some point, they're not going to show him in the teaser just yet.
It'll be cool if he's hinted at through the movie, and we hear every so often how terrified the Rebels are of him. He's like a looming monster essentially!! Then once at the end, when the Rebels hand over the plans to Princess Leia, and they go separate ways from the Princess, Vader arrives on their ship and ****ing MOWS THEM DOWN. Then he turns his attention to the Tantive IV and orders his men to chase after it
I'm very keen to see this CGI Peter Cushing, assuming they can do it right (and it's ILM so I have a lot of faith they can). It'll be so interesting, the fact he'll be in a new movie... 22 years after he passed in 1994. What a world we live in!