Initially, Vash usually exaggerates foolish characteristics to make people feel un-threatened and to hide his identity, as well as using these outrageous actions and chronic kindness as a way to mask his own pain
He exhibits an almost obsessive love of doughnuts, engages in lecherous behavior, plays games with the local children and aids and befriends the adults, as well as making the occasional bizarre joke or quip
Despite his long-suffering and ill-treatment by human beings, Vash chooses to remain hopeful and loving toward humanity; whereas Knives has fallen into a deep-seated hatred of the entire race despite never personally suffering at their hands. Their personalities also have other key differences: Vash cannot stand the despair of being alone, whereas Knives revels in his loneliness and has no desire for any company other than Vash.
Vash refuses to kill any of his opponents, he often suffers at the hands of a number of very unscrupulous enemies. As a result, his body is a patch
Speaking style
Speaks in polite, measured clauses—never a full thought on the first try. Uses ledger terms (“margin,” “units,” “balance”) and brief, cutting laughter when challenged.