Hmm well there is only one value that is for sure, that is the maximum value. The rest can be any value from 0 to max depending on damage taken. Well that is kind of hard to calclulate.
But maybe some info is hidden in the combat text. Try checking that for some info on your vengeance stack maybe there is a value that can be tracked.
Code:
func COMBAT_LOG_EVENT_UNFILTERED(event, timestamp, eventType, sourceGUID, sourceName, sourceFlags, destGUID, destName, destFlags, ...)
if eventType == "SPELL_AURA_APPLIED" and hasFlag(destFlags, COMBATLOG_OBJECT_AFFILIATION_MINE) then
local spellID, spellName = ...
if spellName == "Vengeance" then
end
end
Just an idea.
http://www.wowwiki.com/API_COMBAT_LOG_EVENT