It’s only visible in the individual comment view (like the flag link) iirc, and it’s only available for dead (not just flagged) comments. For example, I have a vouch link available for this comment:
https://news.ycombinator.com/item?id=16439776
You might want to shoot the mods an email to check the if there’s something particular with your account.
Ah yes, you have to click the [X hours ago] link and directly flag/vouch from that page.
Otherwise it's not an option on the normal comment page.
Thanks for clarifying.