For all of those who look forward to see Vader again (as I do), here comes a major spoiler:
Vader is getting his ass kicked by Jyn in the movie. Yes, you heard right. Lord Vader gets his ass whooped by her big time during a fight. How is that even possible? Well, he isn't very much used to his new prosthetic limbs yet, so that's how the writers try to justify this crap. The person in the bacta tank is also Vader/Anakin, recovering from his injuries, which are the reason for getting new prosthetics.
The Star Wars franchise is doomed...
I'm sorry, dude. Absolutely nothing you said is true.
Not used to his prosthetic limbs?
At this point, he has had them for almost twenty years. This movie takes place just before A New Hope.
He has been cyborg for a long, long time.
Also, Jyn does nothing to him.
By the way, people saying (and complaining) that he will be in the film for like ten minutes are correct.
However, the complaining makes no sense.
Why? Simple.
He was in A New Hope for only ten to eleven minutes. The exact time escapes me, but it is truth.