So, tonight I finally resolved on what I hope will be a fun build, tailored for the Dawnguard DLC (I've had for months, it's about time...). Nord: Two-Handed, Archery, Restoration ; no armour in particular, just whatever feels right ; an artistic flair for smithing and minor enchanting, neither of which as 'major' skills, I have no desire to power-game with crafting skills (or with anything else).

A Witchhunter of a sort, a talent for smiting Undead to be specific; general combat is with bows (until Crossbows) and Warhammers, dual wielding restoration spells for anti necromancer / undead crowd control, defence, and eventually offence once I get those nice Sun spells from the Dawnguard... until the magicka runs out. Atronach stone for the most part.

All started very well, got to gaze lovingly at Hadvar for a while, hang out around Riverwood and observe the petty squabbles of the townsfolk. On my first night, after dispatching two bandits at a camp high above the town I cross past the road leading up to Helgen, when two crazed fiends come baying for my blood. With some judicious use of healing magic between, I set about emulsifying their heads in turn with my trusty Iron maul. A worthy effort, but futile in the end.

Well, not quite.

You see, I have since noticed a recurring, inexplicable lethargy, an anxiety that I just can't get rid of... and when some hours later I stepped out of Embershard mine covered in fresh bandit blood, into the newly broken dawn, I swear I felt my eyes burn just a little until they adjusted. I've heard of this before, my old master would make me read 'Immortal Blood' over and over, translations of Galur Rithari's journals. But nothing can compare to the feeling.

As you can imagine, this puts rather a different spin on matters. I could allow the change occur, seek out a cure, etc... the whole self-hating Vampire paradigm. Or, I could throw the baby out with the bath water, abandon my principals and find out if Cyrodiil blood really does taste like sweet-rolls.

I want to do the right thing, but, you know how it is...

Or - shall I propose another option? She (I'm gonna assume by the 'loving gazing' directed at Hadvar that your witchhunter's a she) could hastily find a cure for the disease, ridding herself of it and joining the Dawnguard (since vampires can't enlist), bringing their initiative to its victorious conclusion... only to get infected again shortly after the quest's end. It's at this point that she'd realize there were still plenty of fanged and ghoulish creatures out there to be slain and recognize vampirism as a way of getting her hands on the tools to better drive back the undead scourge. After all, who knows the darkness more thoroughly than one who dwells in the shadows? Meridia's quest, 'The Break of Dawn', would be a great way to kick off that new phase of her life as it features a Daedric prince who despises the undead despite her own... ahem, moral ambiguity. Your witchhunter could feed exclusively on people whose own ethics she wasn't a fan of, or perhaps simply folks who irked her in some way. (Nazeem and Heimskr come to mind.) That way she'd be weakening her foes and strengthening herself all at once!
Or she could sample innocent ladies and gents at her leisure to find out if Cyrodiil blood really does taste like sweet-rolls. (My own vampire likens it to more of a, say, long taffy treat taste.)

I recommend you desperately search for a cure to no avail. You then try to mask your true identity while still fighting for your cause, ever the one to search for the cure in your travels.