nah, I don't think of flag/vouch as the same as upvote/downvote.
Regardless of the constant arguments around it, I use upvote/downvote on opinions I agree/disagree with. I'd use flag/vouch for things that I don't/do think should be on HN at all.
e.g., on a Pokemon forum I can strongly disagree with someone who says squirtel is the better than pikachu because water-type has more advantages that electric-type. I might downvote it, maybe. But if someone posts "Pokemon are stupid, you're all wasting your lives you big losers." ...I'd flag that. Conversely, if I posted how electric-type pokemon are the best and I notice a dead-reply to me explaining very clearly a position supporting water-type pokemon as best, I'd vouch for it. While I might not agree with the argument, I can at least vouch that it shouldn't be dead.