Comments that get autokilled are just [dead], not [flagged] [dead]. If you cannot vouch comments back to life, it's also possible that your vouching privileges were taken away because the mods disagreed with your vouching decisions. When you see something really great that is [dead] and vouching doesn't help, you can try emailing the mods and mention that vouching didn't work. If it's actually great, maybe they'll let you vouch again. (Happened to me.)