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:
Checked it out. No vouch link. Thanks for mentioning it though.
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.
It's possible I'm mistaken then. My apologies. I did not intend for my suggestion to be perceived as a punitive measure had been taken ("which leads me to believe it's been turned off for my user").