Vandalism is the conspicuous defacement or destruction of a structure, a symbol or information against the will of the owner/governing body. In the context of an online community project, it is a usually deliberate attempt to damage the usefulness of content for other viewers.
Useful community content is that which most concisely and accurately reflects the widest range of relevant information on a topic. In the context of a wiki project, intent to reduce relevant information to that of a single viewpoint could be considered vandalism. In keeping with assume good faith, however, it may be possible to look at a one-viewpoint edit as additional information that the user feels should be incorporated or is under-represented. In such a case, it benefits the project to explain more effective methods of bringing this information to light while retaining the existing knowledge in the article. While calling someone a vandal almost always makes the accusation true, extra patience instead can enable one to become a beneficial editor.
Avoiding critical mass
Vandalism usually starts as a form of heavy-handedness which is never effective at getting a point across in a wiki community. We can nip it in the bud by explaining more effective ways of making proposals and working with others. If we alienate the editor, however, vandalism can turn into a form of vengeance. The wiki engine is written to minimize the effect of pranksters, and can even handle a few full blown vengeful vandals, but it cannot handle a vandalism movement that has reached critical mass, a situation where we react to vandalism by assuming bad faith and thereby create more vengeful editors. As a project approaches crisis levels, it becomes harder and more time intensive to assume good faith, but every act of kindness and patience moves the project away from critical mass.