I think that one of the fundamental vices that enables the animals to be susceptible to the pigs' deception is the trust in government/ fear of voicing dissent. From the public executions in chapter 7, the animals have come to see Napoleon's brutality and the consequence for speaking out against his rule. Once Napoleon has made this absolute, it becomes very easy to manipulate the animals into a position of complacency. The animals who speak out are promptly executed, such as the chickens who try to kill Napoleon. It has become evident that there is really no one to speak out against the pigs and Napoleon. It is this reality that makes them vulnerable to deception.
Whether or not this is a vice could be debated. The animals don't gain anything from this, as they are merely seen as means to ends for political satisfaction on the part of the pigs. Yet, it is this trusting nature or the inclination to not say anything because of fear of reproach or political retribution that places them in a position in which they are easily manipulated.