There are several main points in building a Vampire Counts army:
1. Use as many ordinary troops as possible.
2. If possible, take at least two spell casters.
3. Also take a variety of adaptable troops such as Vampire Bats.
4. Make sure that losing your general is not an incredible disaster!
I will organise this article based around these points. Click here to download the text version.
The Vampire Counts army book states that 'Most Undead commanders agree that the army needs to be based around a solid block of Undead infantry,....'. Generally speaking, this statement is true. Some commanders will take this theory literally, and feature a single, huge unit of infantry packed with characters. This flaw will be explained in 'Using Your Army'.
In my opinion, the best thing to take is two or more units, as near equally powerful as possible. While your general's unit will almost always be more powerful, a Vampire Count or a Wraith can bring the other unit/units up to scratch. Don't worry about having a slightly less powerful unit than the others, just remember to support it.
Which type of troops?
My favoured troop type is skeletons, as they can be given extra equipment, and therefore tailored to fight specific roles or even enemies. Zombies, however, provide a cheap alternative if available points are low. Zombies do need to be supported by a good character such as a Vampire, otherwise their low initiative means they will probably be killed before they can attack. Grave Guard are the choice for a general who likes fairly small, elite units.
Size
For skeletons, 20 models is about the right size. More than this can be expensive and unnecessary, if you have the right spells (see magical tactics). When choosing zombies, any number of at least 20 is fine. Their cheap points cost mean that it is worthwhile to have a lot of them. Unfortunately Grave Guard are expensive, and the minimum size I would recommend is 15, preferably 20, as they cannot be raised like other troops. They are tougher, but then again they are a lot of high strength armies out there!
Characters
Each Undead unit needs to be supported by a good character to bolster its fighting strength. My only choice for a champion is a Wraith, but Wight Lords are also good. Skeleton champions really only serve to stop the unit crumbling if the general dies (see later).
Equipment
Skeletons don't really need armour. More often than not you will face enemies with high strength that negates your save, but I like to take shields on my generals regiment to give them a bit of protection in combat. For Undead, spears are a good choice, as their low initiative means they will usually strike last. Good way to give your opponent a surprise! Halberds are useful for a regiment with a good(Vampire) character in it, as they are more likely to be able to attack after the Vampire has cut down the foe!
Simple section this. Take at least two spellcasters, and at least two necromantic spells. That way you can give the important spells to someone survivable (i.e a Vampire).
Anti war-machine
Vampire Bats! In small battles two is fine, but six enables you to attack the rear of enemy units when they are engaged. Another option is Dire Wolves. As long as you keep them away from enemy fire, they can devastate a war machine easily. They are a high risk option though, as they are relatively few, and vulnerable to shooting.
Fast Attack
Fast attack units always need to be supported by other fast attack units. The best choice really is the Black Coach, but Dire Wolves are great too. If you have anough points, Wight Cavalry are pretty good, but you really need two ranks to be safe. Be careful of over-equipping them though. Really this section is down to your army style. My Necrarch army uses no fast unis at all (other than Vampire Bats).
Ghouls
I hate ghouls. Use them by all means, but don't expect much. To be safe, units of bigger than twenty are neccessary, and without characters, they stand no chance except against Empire troops!
Banshees
Always take a Banshee if possible! Nothing more needs to be said other than these things can bring down giants in one roll!
The only way to prevent an immediate loss when your general is killed is to take mostly infantry, with at least one character in each. To prevent your generals unit crumbling, take a low character in that too, so your opponent isn't left free to attack your other units. To save your general, use the Jade Amulet.