Reasons to play Vampire Counts:
Mass-regenerating units
Near-unkillable characters, when properly done
Ultimate tarpits (see first reason)
Awesome screaming power
Bloodlines-Dracula (von Carstein), Sensual and stunning but deadly (Lahmian, also hate men), ancient wizards locked away in towers studying and experimenting (Necracrh), devolved monsters living off stale blood in graveyards (Strioi), martial perfection, and a Master Vampire who cured himself of his thirst (Blood Dragons and Abhorash)
Great spells
Most units can be used as chaff in one way or another, especially dirt-cheap (3-8 point) core
Ethereal chaff
Ethereal light cavalry who can run through enemy units and cause damage
Healing Lore Attribute
Easy to theme
Due to global reach of Vampires, any bloodline or personality can be invented
An Ethereal caster (or he can fly) who uses Death and Vampires, reveals magic items, and heals units by causing wounds
A Chaos Champion hero
A caster with 5 wounds, Vampire Lord profile and bucket loads of potential power/dispel dice, partially from combat
Can be semi-professional, regimented army, or can be a massive, shambling horde, based on character choice
Vampires and Undead, creatures of the night
A spell to increase movement or combat bonus
Those are all the reasons I can think of to play Vampire Counts.