Even if it were powerful enough to belong to a Skywalker, we can't really blame Yoda for not considering Vader a candidate. It doesn't mean he knew whom it belonged to. He felt a really strong force, and felt that it had some good in it. To me it seems like this: Yoda just didn't know who the other hope was. I don't think that this is necessarily true. The counter-argument that I keep seeing is that Yoda was specifically referring to Leia. The ending makes much more sense that way. When Yoda said 'there is another,' I always figured it had to do with Darth Vader. Interesting theory here about Yoda's 'there is another' line in TESB: