Notice that ANH and ESB never say who Anakin’s master was. They only say that Vader was Obi-Wan’s apprentice.
I mean, it’s kind of implied that if Vader is Anakin, and Obi-Wan taught Vader, Obi-Wan probably taught Anakin. So it’s not like they never say that in ESB.
It would be interesting if Yoda was Anakin’s master. In fact, in the Brackett draft of ESB, the Luke/Anakin lightsaber has the coordinates to Dagobah stored on it. This makes me think that Brackett intended for Yoda to be Anakin’s master. After Obi-Wan and Anakin complete their training, Obi-Wan, arrogant in his ability to be a master, trains Vader, and the rest is history. It would also be interesting if Yoda warned Obi-Wan, Anakin, and Vader not to get involved in the Clone Wars, but they ignore Yoda’s warning.
Definitely interesting. I’d love to see something like this fleshed out.