Karma threshold isn’t the only criteria. I’m at ~47k karma and do not have access to vouch functionality. I might be in a sort of permanent semi penalty box (which I take no issue with, not my sandbox), so take with a grain of salt.

for me it's there for [dead] stories, but not [flagged] ones

[dead]: https://news.ycombinator.com/item?id=27397440 (has vouch)

[flagged]: https://news.ycombinator.com/item?id=27396685 (no vouch)

Good point. Same here