Obi-Wan learns the truth of Anakin becoming Darth Vader (referred to by that name *twice* - by the Emperor he personally sees on the security footage - “You have done well, my new apprentice. Now, Lord Vader, go and bring peace to the Empire" - and in follow-up conversation with Yoda that Skywalker is gone and has been "consumed by Darth Vader" - “Twisted by the dark side young Skywalker has become. The boy you trained, gone he is. Consumed by Darth Vader.”
Also they had not met since their last battle per the dialog in Episode 4: A New Hope: "I've been waiting for you Obi-Wan...we meet again, at last. The circle is now complete. When I left you, I was but the learner - now I am the master.
Okay, you are right. I forgot about that particular detail.