Have you ever noticed that some conversations can become more and more heated the longer they go? And perhaps you’ve seen that sense of frustration “blossom” into downright vehemence. Well, when you see that, you know that most likely some non-rational factor or nerve has been touched and is being expressed.
We would distinguish between vehemence and hostility in the following ways. By vehemence we mean the vitriolic expression of a point of view or accusation. hostility 
we mean an expression of a point of view that is threatening in one way or another. Both are typically expressions of non-rational processes at work, but vehemence tends to be emotional and fearful, whereas hostility tends to be dark and abusive.
In the situation of experiencing vehemence, we suggest that you take a deep breath and emotionally step back. Matching vehemence with vehemence doesn’t seem to work persuasively or relationally...oh, and by the way, it’s returning evil with evil. Allow the person to get it out. And if they settle down you might unemotionally (here your tone of voice is very important) help them weigh the pros and cons of their concerns.
If the vehemence and especially hostility doesn’t begin to tone down, you might want to indicate you want to stop and walk away...and then do so.