Ventrilo harassment is a form of trolling over the Ventrilo system.

There are tons of Ventrilo harassment videos all over YouTube and other sites, like http://www.ventriloharassment.org/, it's birthplace.
provide info about who started the site, and the size of the community

Tons of people do this and record it,
please use real numbers

post it on YouTube for the lulz.

And, because it's a trend.
provide data to illustrate the trend.

It's a lot like crank calling with various soundboards and audio, when you think about it. However, it all isn't just two pictures of two people flashing back and forth when someone says something- it's actual text, and clips from movies and shows the audio was taken from. Here are a few of the most viewed, top rated Ventrilo harassment videos.

Ventrilo Harassment – Duke Nukem Style

Ventrilo Harassment – World of Warcraft Nerd

Ventrilo Harassment – Dental Plan

Ventrilo Harassment – Over 9000

Ventrilo Harassment – Darth Vader

Ventrilo Harassment – Gay Bar

Ventrilo Harassment – The Mask

Ventrilo Harassment – Chris Hansen

Ventrilo Harassment – Chris Hansen 2

They've gotten more creative with their videos, even. Now they play the Microsoft error song to annoy people- to make them think their PC is glitching up, or something.
Ventertainment – Sticky Keys

There is much, much more videos. These are just a few. People have their tons of different labels for their videos- Ventertainment, Ventrilo Harassment, some people even make YouTube accounts to be named after their video with only that video on it.