Vandalism is any addition, deletion, or change to content made in a deliberate attempt to reduce the quality of the PlazWiki. The most common type of vandalism is the replacement of existing text with obscenities, page blanking, or the insertion of bad jokes or other nonsense.
Any good-faith effort to improve the PlazWiki, even if misguided or ill-considered, is not vandalism. Apparent bad-faith edits that do not make their bad-faith nature inarguably explicit are not considered vandalism at PlazWiki. For example, adding an opinion once is not vandalism -- it's just not helpful, and should be removed or restated.
Committing vandalism is a violation of the PlazWiki policy; it needs to be spotted, and then dealt with – if you cannot deal with it yourself, you can seek help from others. You will be banned from the PlazWiki.