Vampires fare poorly at 500 pts.
The hero is key, and only has 25pts to spend on Vampire Powers/Items. If they kill your hero, your army will crumble quickly (and 25% of your army is gone with that one model removal). You lose your hero, you likely lose the game.
There are two main options:
Hide the hero
Give the hero protection.
Your army list will revolve around that choice.
The main things you'd want at this level of points is: Summon Ghouls, Flayed Hauberk. That's it. Flayed Hauberk provides the best protection, Summon Ghouls is amazing and lets your level 1 wizard cast spells. If you choose summon ghouls, you have no protection; just T4, and W2, with not a lick of armour or ward save.
My lists for the two approaches:
Vamp Hero (125)
Summon Ghouls, Talisman of Lycni
Ghouls x10 + champ = 88
Ghouls x10 = 80 (vamp goes here)
Grave Guard x15
- Great Weapons, Standard = 207
=500
The Great Weapons are optional, but are one of our only options at this level dealing with High Armour opponents. We have low I, so likely even with HW&S we'd be going last. Step Up can be affected at this low level of points, so the GW aren't as good but still give a necessary tool.
Ghouls w. Champ and Grave Guard are in front, Vamp bunkered behind. Vamp uses 2 dice each turn casting IoN to make the ghouls bigger. If the enemy doesn't have shooting/magic, has the option to leave the unit with a M of 9, or if there's a unit coming that he needs to flee from.
Pros to list: Summon Ghouls is terrific. Each successful casting of IoN will give another 3.5 ghouls, so = 28pts worth of models (more if its the champ). That goes a long way in a low powered game.
Cons: Vampire needs a bunker, which ties up 80 pts of models right off the bat. Even raising 80 pts of models, you break even.
Fighty(ish) list:
Vamp Hero
Flayed Hauberk OR Dread Knight (if they have template weapons, Flayed Hauberk. Regular bows/arrows can't target your Dread Knight Character). Dread Knight gives an extra S4 attack each round (horse), S7 on the charge, and M7 instead of 6.
Ghouls x15 w. Champ = 128pts
Grave Guard x19
- Great Weapons
Depending what you play, you may want to include a standard on the Grave Guard, and possibly the Hellfire banner. But every model counts at this level.
Pros: Vamp should go right on the front lines in whatever unit isn't going up against something really hard. No points tied up in a bunker.
Cons: Vamp is a lvl1 wizard with nothing to cast unless you get a lucky roll. He can summon some zombies until you hit combat. Once you're in combat, 2 dice IoN to heal the Ghouls and Grave Guard.
Personally I think Summon Ghouls is stronger.