Percy is a kid...would he kill Vader if he had the chance? Who wins initiative? If Vader does, it could be over in a flash of his light saber, taking off the boys head...however, Vader's not like that. If he doesn't know the kids power, he would more likely gloat about how useless it was for him to resist...etc. If Percy jumps in the water, it sounds like he's safe there. Perhaps Vader leaves, he has no problem with fleeing if he sees the power this boy can unleash and doesn't see a way to beat him. (of course that's a win for Percy I guess).