I noticed that after activating the ability Body Shot as a thief, there is a period of pause that stops you from preforming any attacks such as Vital Shot. Given that Body Shot and Black Powder have the same cast time, after using Black Powder one can immediately use another attack (Vital Shot) where as after Body Shot exists an awkward pause of inaction.
I am uncertain if this is intended, but it does throw off the feel of the skill a bit.