<p>No, “vehement” is the right answer for reasons already stated in this thread. The author’s tone was not bitter/witty or sarcastic. Vehemence doesn’t imply anger and hostility – it just implies something firm, forceful, emotional, and passionate. You can deny something vehemently, or show vehement support for something, or engage in a vehement debate, and so forth. It carries a connotation of “Damn, this person is serious and cares about their position!”</p>
<p>If you have a caustic tone, you’re coming across as a sarcastic d-bag, generally. Think House from the show of the same name – the way he criticizes people with witty sarcasm is a pretty clear example of caustic tone.</p>