I love playing Calabim and having vampires armies.
The idea is : take cities, feast on them, get stronger vampires ... etc
The only problem is that my economy usually cannot support more cities. So I have to take the city, feast on it until it has a 3 population. And then since vampires cannot feast on cities with less than 3 pop, let the enemy retake it. and take it again but this time raze it.
Is there a reason why vampires can't feast on cities with less than 3 pop and destroy them ? that would be simpler. and it would seem more logical : why would vampire refuse to destroy a city and its puny humans ?
oh and at the end. when the enemy civ is nearly dead. it doesn't have enough units to take the 3 pop undefended cities back ... so I actually have to make a cease fire. give the cities back to him. and then take them back and raze them.
if it's an unfair advantage to get xp from small cities (I don't think so big cities are best to get xp) just let the vampire feast without getting any xp.