The Outer Rim, where Tatoonie is located, is supposed to literally be like the Wild West, in terms of the Star Wars galaxy. No matter what the ruling entity of the galaxy is, they have trouble enforcing in the Outer Rim because it’s so far from the “core” where planets like Coruscant, Alderran (RIP), among others are located.
Recall that in TPM Watto said they wouldn’t accept “Republic credits” there…. So the Republic, which was established for hundreds of years, had little jurisdiction there, let alone the Empire, which had only been around for a decade at this point.
That’s why the Hutts/crime lords essentially rule the Outer Rim… because none of the established governments have cared much to do so themselves.
So TL;DR… it would make sense that there wasn’t much talk or awareness of the Empire’s #1 enforcer/Jedi hunter since the Empire didn’t bother enforcing in the Outer Rim much at at all.
Bail Organa probably was aware that there was a guy called Darth Vader who was hunting Jedi, but would have no way of knowing its Anakin Skywalker. Obi-wan thought he had killed Anakin, and likely told Bail the same thing. Thus, that’s also likely what Obi-Wan told Owen, so he also would likely not make any kind of connection between Anakin and Vader, even if he were aware of Vader.
Also Bail wouldn’t really have any reason to tell Obi-wan there was a badass in a robot suit hunting Jedi, as he was already aware that the Empire was going full-bore hunting Jedi… the “who” is really irrelevant.