Assumptions:
Mana Cost per Second
Vengeance+Fanaticism (speed 8 frames): 33.4
Vengeance+Conviction (speed 10 frames): 26.8
Zeal+Fanaticism (speed 4
frames): 2.5
Zeal+Conviction (speed 5 frames): 2.0
Damage per Mana
Vengeance+Fanaticism: 124
Vengeance+Conviction: 248
Zeal+Fanaticism: 754
Zeal+Conviction: 1508
In summary:
Vengeance deals about 2.2 x more damage than zeal
against PIs, but you run out of mana very quickly. In terms of mana efficiency,
Zeal is about 6 times better than Vengeance. I'd recommend using Zeal when you
have problems regaining mana. Most paladins do, because they rely mainly on mana
leech to regain mana. Unfortunately mana leech doesn't work against PIs.
However, if you somehow find enough mana to support Vengeance, go with it. It
deals a lot more damage than Zeal vs. PIs.