Well, "vindictive" doesn't mean "liar". Someone vindictive could nevertheless be honest to a fault; in fact, their honesty could stem from the same source as the vindictiveness against someone who had been dishonest.

It's a kind of guilt-by-association reasoning: if someone has X bad quality, they likely have Y bad quality also. The boy who stole the loaf of bread from the baker must be the one who assaulted the farmer's daughter.