Luke Wasn't Alone: a "Multiple Jedi" Theory

I'd add that there's a fair amount of wiggle room in Yoda's "No, there is another." line in ESB. Sure, the most obvious interpretation is that he's talking about Leia, but let's assume you're right in thinking that Luke was groomed for Vader-confronting because he was the most likely to get through to the better bits of Anakin still buried in the armor. Hypothesis: Vader's enough of a narcissist that the son who reminds him of an innocent younger version of himself stood a much better chance of winning him over than would the smartass daughter (1) who physically resembles the wife who turned on him, (2) who challenged him in the Senate until the Senate no longer existed, and (3) whom he'd actually already brutally interrogated all the way back in ANH. In that case, Leia's mostly irrelevant to Yoda's schemes, no matter how naturally Force-sensitive she might be, and he may instead be referring to someone capable of taking a more traditional approach - or, heck, Ahsoka Tano.