Lets start with: 500 Points Vamps is pretty tough. ;-)
What works best at this low points is a
Necro, some core and a big unit of crypt horrors.
But back to your list:
1) Your vampire is a week spot (which is normal at 500 points). The wizard level is not sufficient to really cast anything and he is missing crucial damage output. But the biggest problem is the defense: He can not take a “look out, sir” roll as he is the only mounted miniature in your army and the protection is not sufficient. After he is dead your army will just crumble.
2) You have a lot of points in core. Vamps should keep core units to a minimum. In my opinion the zombie units are not big enough, because you will not cast to many IoN's to grow the units.
3) Grave guard is too small. They attack only at I3 and will be the secondary target (after the vamp) for shooting.
Suggestions for your list:
1) Reduce your core. Use the points for more Grave Guards. Maybe even add GW (Otherwise your vamp is the only one who can cut threw armor).
2) Either swap the GGs for BKs or unmount the vamp.
Hope that helps.
