I've never met anyone who actually got a tip before. I suspect Vocal makes it a feature since making money from reads is such a remote possiblity for most writers, they want to keep exploiting them so they hold that out as a carrot. The challenges are the same thing. They're often sponsored by an advertiser (which is actually an ad), so Vocal isn't even really paying the writers, the sponsor is. It's nice of you to tip when you're not a subscriber, but usually when you are not, you're data is being mined. You don't get anything for nothing on these platforms.