What if, instead of an upvote/downvote button, it was a scale ...
I know it's just semantics, but if at the bottom of the post, there was a slider.
If you slide/swipe to the left, "not useful to me", if you slide/swipe to the right, "useful to me". The further you slide to one direction or the other, the more positive/negative vests.
By default, make it a 1-10 scale, with the option of going more granular.