We learned plenty.
KENOBI: A young Jedi named Darth Vader, who was a pupil of mine until he turned to evil, helped the Empire hunt down and destroy the Jedi knights. He betrayed and murdered your father.VADER: He is here.
TARKIN: Obi-Wan Kenobi? What makes you think so?
VADER: A tremor in the Force. The last time I felt it was in the presence of my old master.
TARKIN: Surely he must be dead by now.
VADER: Don't underestimate the Force.
TARKIN: The Jedi are extinct. Their fire has gone out of the universe. You, my friend, are all that's left of their religion.
Facts about Vader (1977. edition):
1. He is evil and ruthless.
2. He works for the Empire, but his position in the imperial hierarchy isn't specified, although only one man (Tarkin) appears to outrank him on the Death Star.
3. He used to be flesh and blood, a Jedi, who at some point became a cyborg (More machine than a man). How, when and why? Wasn't specified.
4. He was Kenobi's pupil, a good friend, and an excellent pilot
7. A traitor and mass murderer of his own Jedi brethren.
It takes about 15 seconds to read this list, so how exactly does this qualify as "plenty"?
The amount of time it takes to read this incomplete and erroneous list has nothing to do with what the audience comes to know about the character of Vader in Star Wars.