A Vampire Thrall that has been upgraded to be your Battle Standard Bearer replaces the "Hold your Ground" rule given in the Warhammer: the Old World rulebook with the version given below:

"Hold Your Ground"

Friendly units within the Battle Standard Bearer's Command range may re-roll any failed Leadership test. In addition, friendly units within the Battle Standard Bearer's Command range reduce the number of Wounds lost due to the Unstable special rule by D3.

Note that this is not cumulative with the Indomitable (X) special rule. If a unit is affected by both, use the highest value.