It is realy Up really up to you. If you feel that you won't get hs without the vampirism, go for critical +3.
You have to consider that vampirism can be used on the armor too, but crit can only be used on the weapon.
Moreover since critical will help you to increase the damage, the vampirism of the armor will 'heal' you more in each hit, not as much as you would if u had the 2 vampirism, but enough good. Hitting more will make you get more exp/h and loot/hour aswell.as well.