Chrille said:
The problem comes when Obi-Wan calls Vader his pupil, and says that Vader killed Luke's father. A prequel viewer would know that Obi-Wan never had a student called Vader, and that indeed Anakin was his only student. Wouldn't that make the viewer figure out Vader's identity?

When we meet Anakin in ROTS he is a Jedi Knight, Obi-Wan could have had other pupils after Anakin and a few bread crumbs could be left at the scene like having Palpatine refer to Vader before Anakin's fall (because Palpatine has already decided that's what his new apprentice will be called before he has fallen).