Which word best describes a person who annoys or troubles others frequently?

The term "vex" accurately captures the essence of someone who frequently annoys or troubles others. To vex someone means to cause them irritation or distress, which aligns perfectly with the behavior described in the question. When someone vexes others, they are often engaging in actions or displaying characteristics that are bothersome or disruptive, making the word a fitting choice for this context.

In contrast, the other options do not convey the idea of annoyance or trouble. A zealot refers to someone who is fervently devoted, often excessively so, to a cause and doesn't inherently pertain to causing annoyance. Voracious describes someone who has an insatiable appetite or eagerness, typically related to food or learning, rather than a tendency to irritate. Vivacious denotes someone who is lively and animated, which conveys positive energy rather than troublesome behavior. Thus, while all the words have their own distinct meanings, "vex" stands out as the most appropriate choice for describing someone who frequently annoys or troubles others.