Due the increase cost of regular abilities a vamp tank shouldn't be viable anymore.
But what with the 30% damage migitation, the drain and stun skill and the ultimate you get as a vampire? Doesn't it pay off? Cant you easily build around the -8% regular ability cost? (as a stage 3 vampire).
Shortly, I'm not convinced its a bad choice for a tank. So I would like to read your opinions and maybe see some tank/vamp builds.