Vandalism is any addition, deletion, or change to content made in a deliberate attempt to reduce the quality of the ARRSEpedia. The most common type of vandalism is the replacement of existing text with obscenities, page blanking, or the insertion of dullardy. Vandalism is a typical form of behaviour demonstrated by Sock Puppets. Fortunately, this kind of vandalism is usually easy to spot.
Any good-faith effort to improve the ARRSEpedia, even if misguided or ill-considered, is not vandalism. Apparent bad-faith edits that do not make their bad-faith nature inarguably explicit should not considered vandalism. 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 ARRSE Rules; it needs to be spotted, and then dealt with. If you cannot deal with it yourself, you can seek Moderator support and the DeleteMeister will be dispatched to deal with it.