I love vampires too
Earlier this evening I fought Xeona at Castle Nexy. This is my first game at Impossible, but having beaten Karador and the Demon leader I thought Nexy wouldn't be too bad for my gallant Warrior, Sir Brand. It turned out to be disastrous. My skeleton archers got off one shot, then were turned into sheep and teleported (exchanged) deep into enemy lines by demonesses; that's the last I saw of them, as they were chopped to bits by demons. My Lake Fairies were incinerated. (Why I brought such delicate flowers to Demonland I'll never know). My 9 black dragons were not very useful (probably because demons are partly immune to fire--again, I wasn't thinking very clearly when I planned this...) My brave Knights were slowly reduced to scrap metal by incessant attacks from all sides (although this did permit one great circle attack which I think hit five enemy units). My side did some damage, of course, and Time Back and Ice Orbs helped, but I was sure I was going to lose. But my 130 (or so) Ancient Vampires... the last half of the battle consisted of these Draculas sitting in a corner and replacing themselves (with the help of Magic Spring) a bit faster than the enemy (including about 150 Hunters firing into them every turn) could kill them. Gradually the tide turned. Sir Brand won his battle. True, the losses were a bit heavy--I lost 5,659 troops (mainly Lake Fairies) to Xeona's 1,291 [I keep track of losses] but, by golly, I got that key. And all my vampires survived. (Nobody else did, except for some skeleton archers replaced by Necromancy after the battle.)
Ancient Vampires are just great. (I have the Cloak of Shadows and Vampire Ring, too, which helps).
(Maybe I should add that most of my battles have been a little more skilful. This one was not my finest hour.)