"Vandalizing" in the Wikipedia context means editing pages to add stupid, offensive, trollish, or deliberately incorrect content. It happens all the time, which is why some pages (mainly those about controversial topics) have additional protections to make it harder for untrusted people to edit them.
It is amazing how frequently Wikipedia is vandalized. It happens several times per second. Much of it is removed automatically by bots that will roll back edits which contain strings like "qeqeqeqewqeerrttt". On weekends, you can expect something like "List of Sluts" to appear on the articles for many highschools - they will disappear within a minute or so.
You can configure Wikipedia to send you an email whenever a certain article is edited. I monitor several thousand articles that way, and remove malicious edits. Lots of editors do that, and vandalism is usually cleaned up very quickly.
It's a long story, I was a legitimate editor for a about a year and just built up a grudge which turned into making more unhelpful edits until I got banned. Wikipedia is not a good environment for when things go wrong.
