Check "Enabled", Buff/Debuff in the upper left dropdown, spells to track "Vengeance", check "player", check variable text and - if you need/want a timer - check the timer.
And don't forget to click "OK" in the lower right corner.
I use Weakauras and just made a large Vengeance icon that pops up to the right of my unit frame and lists how much Vengeance I have at any given time. It's super useful as you can accurately see how much AP you are gaining and losing by the second. If there's any interest I would be happy to post it when I get home.