What character traits in Fortunato make him fall prey to Montresor?

Fortunato is proud and is vain about his wine-tasting abilities, both traits that make him susceptible to flattery. Montresor lays it on thick, saying,

I was silly enough to pay the full Amontillado price without consulting you in the matter. You were not to be found, and I was fearful of losing a bargain.

It never occurs to Fortunato that Montresor might be mocking him when he says that. Fortunato also dismisses the idea that Luchesi could discern if the wine were a true amontillado. He seems to think he, Fortunato, is the only one capable of making that judgment. He will later say of Luchesi,

He is an ignoramus.

Of course, the irony is that at the point he says this, Fortunato is the ignoramus who has fallen into Montresor's trap.
Fortunato also becomes prey to Montresor because he thinks he is superior to him. For example, when Montresor misses a secret sign of the freemasons, then insists he is a mason, Fortunato treats that idea as ridiculous:

You? Impossible! A mason?

It never occurs to Fortunato that Montresor could outsmart him or that he could be using the word "mason" in a different sense. For that reason, Fortunato comes across as trusting and gullible. He is prey because he is too easily lured into danger by appeals to his vanity.

It is mostly Fortunato's pride and arrogance that renders him vulnerable to Montresor's plans.  Montresor knows that Fortunato will not be able to resist the implication that Luchesi is as good a connoisseur of wine as Fortunato, and that Fortunato will insist on accompanying Montresor to his vaults to see the wine Montresor claims to have purchased.  Further, Montresor knows that Fortunato will not be able to resist the opportunity to tell Montresor that he made a mistake and paid far too much for wine that couldn't possible be the rare Amontillado.  Moreover, Montresor knows that, despite his repeated warnings to Fortunato that the vaults are too damp, especially given that Fortunato has a cold, Fortunato will proudly insist that he is fine, that the damp is nothing.  In this way, then, Montresor knows that he can manipulate Fortunato's pride to be Fortunato's downfall.