There's a difference between instructions and advice.
Would you say that "be brave" is bad advice? Or "don't trust Littlefinger?"
Vague doesn't necessarily = unhelpful or wrong.
Sansa told Jon about Ramsay and Rickon; he choose to disregard her words of warning and try to save Rickon and face Ramsay head on, on Ramsay's terms, anyway. It's that simple.
Sansa knew more about the enemy than they did. Not more about strategy or tactics, but more about who exactly they were dealing with. Again, it's that simple.
She could have tried to guess, but no, you can't really predict what a trickster is going to do anymore than you can predict what a guy like Littlefinger is going to do. You just know, from experience of them, that they're going to try and trick you. That they're not to be underestimated. That Rickon was as good as dead already. All Jon had to do is apply what she said to the fairly obvious situation that arose: do I try to save Rickon? Do I rush forward to fight? No, I think Sansa was right after all; this is Ramsay trying to trap me.
I'm not denying that Sansa made a mistake too. I'm saying they both did. Still, a lot of people seem to be biased in Jon's favour. I give him credit for fighting so hard, but you do remember that he was losing, right? You can try to pin that on Sansa if you want, but ultimately, Jon, the leader, made a bad decision and disregarded good advice.