I was playing around with the damage meter mod Recount today. It's pretty slick actually, and even has pie charts.
In any case, I fired up Seal of Vengeance while working on the quest "Banish the Demons" in Ogri'la. I was comparing the newly fixed SoV with Seal of Righteousness. I have to say that I really don't understand SoV. It seems to be doing far less damage than SoR. According to the meters, I averaged around 215 DPS when using SoV, and 290 DPS with SoR. That's a huge difference!
The Horde seal, Seal of Blood, at least fills a different niche than any other damage Seal. It provides consistent damage based on Attack Power. Seal of Command provides burst damage based on Attack Power, while SoV and SoR both seem to provide consistent damage based on Spell Power.
SoV and SoR both occupy the same niche. As such, I don't really see any case where you would use SoV over SoR. And because my weapon was fast, SoV was inconsistent in its application, and would sometimes even just drop off the target, even when I was just auto-attacking.
I'm sure that SoV could be buffed to be better than SoR, but even then that creates a problem. SoV is either better or worse than SoR. You only ever need to use one Seal.
Quite honestly, I think Blizzard should just drop Seal of Vengeance and give all paladins Seal of Blood. SoR already fills SoV's niche, and Seal of Blood would give all paladins another weapon in their arsenal.