Wight/Wraith
Attack: 7/7
Defense: 7/7
Hit Points: 18/18
Damage: 3-5/3-5
Speed: 5/7
Movement: Flight
Cost: 200/230
Special Abilities: Heal wounded/Drain enemy spell points, heal wounded

The wight is a useless unit, and the wraith nearly so. You don't ever need to build the wight dwelling as requisite in order to build higher-level dwellings. In most cases, then, you're better off using the money saved to build other structures. The wight, in particular, is slow and weak. The top unit of the wight and wraith stacks heals its wounds at the end of the round, so your stack is always at maximum hit points each round - however, their low hit points offset the usefulness of this ability. The wraith also gains the ability to drain two spell points from the enemy spellcaster per turn, which is useful, but are you going to spend the money just for that ability? Once you have built vampire, lich, and black knight dwellings, then you can consider the wight dwelling and upgrade to wraiths. Then create two stacks and bring them along to continually add four spell points to your pool every round. Consider very carefully whether you want to bother using wraiths at all. The answer should usually be no.

Vampire/Vampire Lord
Attack: 10/10
Defense: 9/10
Hit Points: 30/40
Damage: 5-8/5-8
Speed: 6/9
Movement: Flight
Cost: 360/500
Special Abilities: No retaliation/Resurrect dead vampires, no retaliation

The vampire is a bad unit: slow and weak. However, the upgraded vampire lord is very powerful and jumps up in value considerably. That is reflected in its price; it is the costliest fourth-level upgraded unit in the game. Vampires are necessary because they give the necropolis much needed range. The town has no ranged units until the fifth-level lich, and the vampire lord is the first flying unit with any kind of useful speed.

Vampires are weak because their speed and flight usually mean they get surrounded and bludgeoned by enemy units. Their low hit points contribute to their quick demise. However, once you upgrade to vampire lords, that changes. Unfortunately, the vampire dwelling upgrade is expensive and requires the skeleton transformer and the necromancy amplifier, thus delaying the upgrade. However, once you do upgrade the vampire, it becomes immensely useful, as it is able to resurrect units from its stack by attacking enemy units. Every time you attack, the amount of damage you do is applied towards resurrecting fallen vampire lords, although the resurrected number can never exceed the original total. (Note, however, that the vampire lord life draining ability does not work against enemies without life force, such as gargoyles or enemy undead.) So if you cause 80 hit points of damage, 80 hit points of vampire lords are resurrected, bringing two vampires back to life. If you have a large group of vampire lords, this ability is extremely useful. Now combine that ability with the lich's cloud of death and you have a lethal combo. The vampire lord draws enemies around it and can continue to stay alive by draining life from surrounding enemies. The lich then lobs the death cloud into the vampire lord's midst and damages all creatures that are trying to kill the vampire lords. Casting counterstrike and stone skin only increases the effectiveness of this combination.

Vampire to vampire lord should be your third upgrade after skeletons and liches (vampire lords are more important than power liches, but the lich upgrade is easier to come by).