Here is my summary (with references to the actual license and license FAQ) to the licensing of wikis using CC-BY-SA 3.0 (Unported).

According to the license, the content of the Wiki's ("The Work") are considered a "Collection" (item 1(b)). This is the context in which the license takes effect. This also applies to the License Grant to reproduce the work.

The license to reuse "The Work" will automatically terminate upon any breach of the license (item 7(a)). This termination applies only to the party that violated the license, so if a vandal removes an attribution notice, only they would lose their license. ref If you lose your rights to reuse the work, you can only get them back if the licensor gives permission. ref

The license is the entire agreement, there are no addtional terms or implied conditions. (Item 8e).

Since the license is unported, it is written to apply to as many international jurisdictional issues as possible. ref

There may be some jurisdictions in which certain parts of the license may be unenforceable due to existing laws. ref

The jurisiction that applies to the license is the jurisdiction which the license is sought to be enforced in (jtem 8f). If the jurisdiction has a standard set of copyright rights that are not included in the license, then they will be included when sought to be enforced in that jurisdisdiction (item 8f)

But due to the nature of wikis being edited worldwide, and the license being specificly geared to an international stage, it is very unlikely that the juridictions of all your contributors will all find a specific part unenforceable. Edit: Apparently our legal juridiction is in Callifornia according to ToU,

License Errors

And of course according to Wikia, "We take licensing very seriously". Which means none of this applies until they say it does.