[yj589794]

OK, I've done some more testing and here are the results:

A) GetSpellAuraDescription(spellID) does not exist
B) The combat log does not provide any useful information (Vengeance does not appear to be added to the combat log )
C) UnitAura does not provide any more information about the Vengeance buff, the returned parameters are:

1. Vengeance (name)
2. "" (rank)
3. Interface\Icons\Spell_Shadow_Charm (icon)
4. 0 (count)
5. "" (dispelType)
6. 0 (duration)
7. 0 (expires)
8. player (caster)
9. <nil> (isStealable)
10. <nil> (shouldConsolidate)
11. 76691 (spellID)
12. <nil>
13. <nil>
14. <nil>
15. <nil>

===========================

So, it looks like tooltip scanning is the only way to get the AP buff value from Vengeance.

I've created an oUF element as proof-of-concept : http://github.com/Evilpaul/oUF_VengeanceBar

layout code : http://gist.github.com/592615


I'm not sure how much use this will be, but somebody might want to try it out