@the-duressed Good questions. Let's discuss Vampiric Tutor first.
To begin with, Vampiric Tutor is actively bad against the 25% of the metagame running Sphere effects. That's not necessarily a reason to avoid it, of course -- the deck runs Missteps, Misdirection, and Flusterstorm, which are even worse against Shops and Eldrazi. But if you have a card you will board out 25%+ of matches, it had better pull its weight in the other matches.
So let's talk about the control matchup. Oath wins against blue decks in two ways: (a) quickly deploying the combo before the opponent can set up defenses; (b) treading water while applying a steady stream of must-counter threats, until something slips through.
In both cases, Vampiric Tutor is lackluster. In the former case, where you have the initiative, Vampiric Tutor wastes a turn, allowing the opponent to deploy annoying countermeasures like Grafdigger's Cage or Containment Priest that neutralize the Oath combo. Of course, that's only if the Tutor doesn't just get Misstepped.
The latter case is even worse: if you are behind on board, facing a rampaging Monk army, active Jace, etc, and it becomes time to shit or get off the pot, the last thing you want to draw is Vampiric Tutor. Vamp -> Oath -> Griselbrand is an excruciatingly slow chain if your opponent has an even remotely effective clock. Weak players will let you Vampiric for Forbidden Orchard or Abrupt Decay when you need them, allowing you to steal games, but strong opponents will simply leverage their superior card advantage against you and just counter the tutor.
It's possible to make the Tutor more effective, but this requires adding cards to the deck that can counteract Vampiric's miserable tempo and card disadvantage: Probes, Tops, Gushes, etc. It's possible to build Oath this way, but that's not the decklist in the OP.
Now, as for Ponder: I don't think it's garbage, but I do think it's worse than Preordain in an Oath deck, due to the number dead cards in the deck, which can be make Ponder very awkward if you don't have an uncracked Fetchland available. Ponder is better when you have an excellent card third from the top, and a mediocre card in the top two that you might be tempted to keep with Preordain. Certainly I don't think replacing one Preordain with a Ponder would be a huge mistake.