Maybe this sounds weird, but before the prequels I think I had the impression Yoda just always lived on Dagobah. The Emperor and Vader didn't go after him because they didn't ever meet him and didn't know where he was (Vader only says "Obi-Wan has taught you well" and while Yoda does mention things about Anakin's personality he could have just been watching through the Force like he did with Luke). It seemed like there could be two kinds of Jedi, maybe. Some might be gurus who sat at the top of mountains contemplating the universe, while others were more active, like samurai or ronin. Yoda was a guru, while Ben was a general (in the Alderaan navy during the Clone Wars, it seemed). Not that this is particularly relevant to how things were actually portrayed in the actual prequels.