A vampire mage is a new monster in SLASH'EM (and also a deferred feature in nethack). It is considerably stronger than a vampire lord, as well as being able to cast spells. Note that it requires at least a +3 weapon to damage them.
Vampire mages are excellent at wreaking havoc in the later levels of SLASH'EM. Since they are not covetous, they will not warp to the up staircase when hit, but instead will simply flee. As long as they have you in their sight, though, they will continue to summon nasties, and the ring of monsters they will create around you can make it nearly impossible to reach them, particularly on open, no-teleport levels such as Juiblex's swamp or Moloch's Sanctum. It is not uncommon for them to have filled half the level with nasties in such places before you can destroy them. They should therefore be destroyed with extreme prejudice, even in preference to arch-liches, since the latter will flee to the up staircase and (likely) out of sight of you when hit even a little bit. They are an excellent target for uncursed genocide.
Vampire lords will grow up into Vampire mages at level 20. This makes taming a vampire much more attractive in SLASH'EM, as a relatively weak vampire will quickly grow into a level 30 terror. Vampire mages are nearly immune to monster attacks, due to regeneration, healing spells, low base AC plus the ability to wear armor, and requiring a +3 weapon to take any damage at all, making them excellent pets as long as they have not been genocided. In addition, vampires in SLASH'EM can drain corpses; this will increase their tameness, making it far easier to prevent them from going feral. They also will never become hungry; they simply drain any corpse they find. Unfortunately, vampire mages are capable of spontaneously rebelling, particularly if you are much weaker than them and/or you abuse them.
Note that if you want to give your vampire mage armor, you will need to make sure it is not blessed. They will avoid blessed items as most pets will avoid cursed items.