Bonasera was pretty clueless when it came to Vito.
First,he insults him by asking how much Vito would charge to avenge his daughter's assault, as if he was a goon for hire.
To make it worse,he does it in front of other people,and on Connie's wedding day.
In the novel, when Bonasera gets the call from Tom,he conjures up all kinds of scenarios for the "service" he is to repay.
He thinks that he will be asked to get rid of a body, or somehow be forced into doing "Mafia" business.
He gets really worked up.
Turns out that all Vito was asking was for Bonasera to do what he does every day,namely prepare a body for viewing.
His "feeling of utmost despair" was unfounded,but understandable,given his preconceived notions about Vito.