Traditional karma scores, star counts, etc, are mostly just counters. I can see that a bunch of people upvoted, but these days it's very easy for most of those votes to come from bots or spam farms.
The important difference that I see with Vouch is not just that I'm incrementing a counter when I vouch for you, but that I am publicly telling the world "you can trust this person". And if you turn out to be untrustworthy, that will cost me something in a much more meaningful way than if some Github project that I starred turns out to be untrustworthy. If my reputation stands to suffer from being careless in what I vouch for, then I have a stronger incentive to verify your trustworthiness before I vouch for you, AND I have an ongoing incentive to discourage you from abusing the trust you've been given.