Yes, but a Necromancer raises vampires... after the battle, and then you need to see who will be more numerous;
A vampire is a flying unit, and it doesn't seem to affect them.

1) It only raises them during a battle with a 4th-level unit.
Besides, you also need to upgrade Necromancy, which takes extra levels, which isn't always good, and not always necessary.

2) What difference does it make if it flies or not?
It slows them down, of course.
The confusion might be in the following: there are spells that slow down units in both Haven and Necropolis magic, but in the first case, the origin of the creature doesn't matter, and in the second case, it only affects living creatures (by the way, this is called "fatigue").

Also, Haven (introduced in Equilibrium) has a spell to slow down only flying units (clip wings).

P.S. The option of slowing down vampires and then shooting them with arrows is quite good, especially if you manage to fly around them.