I see a lot of substantive comments that are grey; quite often there are dead comments that seem pretty good too.
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.
Otherwise it's not an option on the normal comment page.
Thanks for clarifying.
You need to click the [x time ago] link to see the flag/vouch option.
EDIT: The post was dead at the time I checked, but it's possible it still shows as dead after it has been deleted, which I assume removes the vouch option.