For PvE there is only one reason to keep vampirism on a magsorc dd: the passive unnatural recovery. This passive grants 10% magicka and stamina recovery and you only need to be vampirism stage 2 to unlock it. In stage two the additional fire damage is not a big deal, therefore keeping your character at stage two through feeding or bloody mara drinks will maybe help you out sustaining. This counts for PvE end content.
In PvP it is a similar issue, but fire damage is a very common damage type there. So the disadvantages are even higher there. Many PvP-magsorcs do not use vampirism there.