I strongly recommend Archery based on the last quest in the Dawnguard storyline. You'll have to use*SPOILER ALERT* a unique bow called Auriel's Bow to kill Harkon, who is actually pretty damn hard to kill. This is because he moves pretty damn fast. The best way to overcome this problem is getting the Steady Hand perk. It creates a Slow Time effect, making it a lot easier to aim. To add to how frustrating it is to kill him, he will utilize one of the powers in the Vampire Lord Perk tree called Mist Form, which behaves exactly like the Become Etheral Shout. As for the rest of my suggestions, they're there just for the other quests and the vampire hunting experience. I would also invest in Restoration. The Turn Undead spells and new spells Sun's Fire, Vampire's Drain, and Stendarr's Aura are a definite must-have for any Paladin-based character. Sneak itself is also a good skill to major in, mainly because it makes it easier to kill vampires one by one. Also, you may need to rely on it for the Dawnguard Radiant quests. Armor type is more of a personal preference, but Light Armor does make sneaking easier early on. Other than that, the rest is entirely up to you.