I started playing back when Mirage was released and eventually took a break shortly after Prophecy. I have recently got back into the game after Conflux. I decided I wanted to build a White deck, but I haven't had much luck against others. Now that Zendikar has come out I needed a change of deck type. Right now I am having trouble with the most common types of decks today, mostly Vampires. I want this deck to do well against Vampires primarily since that is mostly what I go against. So here is the deck list.
 
24 Creatures
x4 Deft Duelist
x4 Meddling Mage
x4 Palace Guard
x4 Wall of Denial
x4 White Knight
 
16 Spells
x4 Archive Trap
x4 Honor of the Pure
x4 Luminarch Ascension
x4 Path to Exile
x4 Unsummon
 
20 Lands
x6 Island
x6 Plains
x4 Kabira Crossroads
x4 Sejiri Refuge
 
Sideboard
x4 Celestial Purge
x1 Iona, Shield of Emeria
x2 Solemn Offering: Just incase.
x2 Sunspring Expedition
x2 Gwafa Hazid, Profiteer
x4 Pacifism
 
Any help is appreciated.

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I like your selection of creatures as far as dealing with Vampires goes. Deft Duelist and Wall of Denial both have shroud, and White Knight has protection from black. The weak link here is Palace Guard. Sure, it seems good in a Luminarch Ascension deck, but remember-- a monoblack deck's removal is likely going to be Doom Blade (or Feast in Blood, for Vampires) and Tendrils of Corruption, which will always do at least four damage since the monoblack player needs at least four Swamps to be able to cast it at all. Sure, he may run a few Gargoyle Castle or Piranha Marsh in his deck, but a deck that runs Tendrils will know it wants to maximize its Swamp count anyway.

What I'm trying to say is that when so many of your creatures are immune to black's kill spells, the one that isn't is going to immediately eat one wen it hits play. And Palace Guard doesn't even do a very good job of holding back Vampires, since most of them run Vampire Nighthawk, which has flying. Take out Palace Guard.

If you really want a way to get quest counters onto your Ascension, I recommend Ajani Goldmane. His second ability will not just pump up all your creatures, but give them vigilance and let you leave them all up to block while still swinging with your Knights and such. Also, once Goldmane hits the table, suddenly your opponent may decide he'd rather attack Ajani instead of you, or sack his Vampire Hexmage to dispose of the planeswalker's counters instead of the Ascension's. It's a good way to give your opponent bigger priorities than the possibility of you getting 4/4 flyers for 1W each, not to mention powering up your creatures and pushing through more damage.

I'm also completely perplexed at your use of Archive Trap. You're not a mill deck. Why do you want to take chunks of their library off the top? It won't benefit you. You'd be better off with something that actually deals with something that threatens you. I'd say Negate, as a sort of "catch-all", since you've already got ways to kill creatures. Or maybe Ponder, to help with card selection. Or Oblivion Ring, which is just as good of a catch-all as Negate, plus being more creature removal. Or perhaps Kiss of the Amesha, helping you bounce back from a bad situation with both extra life and extra cards, or solidify a winning position?

Pardon me, I'm rambling. If it's Vampires you want to be able to beat, you have to know the lynchpins on which they rest. The strongest card in any Vampire deck is Vampire Nocturnus, which should be your number one Path to Exile/Meddling Mage target. Number two is Vampire Nighthawk, since only Wall of Denial can block it, and there's nothing in your deck that can kill it if it blocks. Maybe what you really need is a bigger creature? Or maybe Trusty Machete? A White Knight that picks up that for its sword would kill pretty much any Vampire in combat, or simply swing for an unblockable 4 damage. But with so much shroud in your deck, the Machete could easily be a dead card.

Actually, now that I think of it, Arrow Volley Trap could easily take out three of your opponent's Vampires in one shot, if you feel the need for a one-sided Wrath. And who doesn't?

Sometimes just listing off possibilities is the best part of my job.

Good luck!

~BMoor