Please explain what Shaman skill you think of, (though slightly offtopic), I'm a bit ignorant of shaman skills.
Vengeance reduces by 10% continuously(!), so consider cooldowns of skills, please.
Do we have reason to believe those damage reduction skills (lethargy, vengeance, gr. stench, etc.) do not stack?
Except that the -25% thing from sins don't work on raid bosses ever.
Also, only one of each kind works, of course (meaning only 1 vengeance stack. Otherwise every boss's damage would be 0% with 10 Guardians next to him.)
I've never run tests on that on raid bosses though.