It is interesting to note that the same, undead(negative energy) blood that allows the Half-Vampire to live longer also weakens the creatures overall health. So in essence the living blood of the creature's mother both harms and strengthens the being. It harms the creature by "giving" it mortality but also strengthens it with the blessing of living, moving blood allowing the creature to walk in the daylight and negating some of the other effects vampirism has upon a creature's body.

Classes permitted:
The avenger sub-class or the thief class.  Half-Vampires can only be these because these classes help them attain the abilities they need to vanquish the creatures whose dark blood they carry.

Personality:
Half-Vampires are individuals who are obsessed with killing the dread undead whose blood they carry.  This is due to the internal struggle that they must constantly battle in order to stay good of heart.  They feel that if they can rid the world of these creatures then maybe they can gain a permanent hold on their humanity.  They sometimes seek out vampires because of the pain and suffering their fathers caused their mortal mothers (this type of half-vampire tends to be from the avenger class due to the revenge they seek on behalf of their mothers).  These creatures carry out their daily lives constantly battling the internal desire to feed upon human blood and become the creatures they so hate. 

Ability Scores:
Half-Vampires add a +1 to their starting strength and charisma.  This is due to the side effects that the negative energy plane has created on their father's seed and the mysterious animalistic charisma that arises from their feral-like features.  But half-vampires suffer a -2 to their initial constitution due to the weakened blood that courses through their veins,
Half-Vampires can have a maximum strength of 19, a maximum charisma of 18, and a maximum constitution of 16.

Special Abilities:
+2 bonus on saves involving sleep or charm, can opt to use "natural weaponry" meaning the half-vampire can claw, claw, bite( the damage is  1d4+2/1d4+2/ 1d2), if the half-vampire chooses to sleep in a coffin, they receive the following:
The half-vampire needs to sleep for only 4 hours to be fully rested and upon wakening gains 2 hit points of health instead of the traditional 1.

Special hindrances:
Half-Vampires have unusually long finger nails, which is why they can do claw damage( it should also be noted that these beings have longer incisors than regular humans, although not so long as to make them stand out).  To the local populace, this could cause mixed reactions.  The Dm should feel free to penalize the player's reaction adjustment as they see fit.  Secondly, half-vampires have an extreme dislike for mirrors, garlic, and running water.  These items don't cause the half-vampire damage in any way, they are just found to be repulsive by the half-vampire. 
Another hindrance a half-vampire receives is that they can only hold holy symbols for 2d4 rounds at a time, after this the holy symbol starts to burn their skin and they take damage.  This is caused by the half-vampire's unholy blood.  Lastly, a half-vampire takes half damage from holy water.  This is due to the reaction of the half-vampire's vampiric blood to the sanctified water.