From what I can tell, default XenForo has relevance-based searches disabled. However, if you have an add-on installed which provides a SourceHandler that supports relevance, then XenForo will perform relevance-based searches. VaultWiki's search templates already have the relevance option when XenForo provides the standard $supportsRelevance viewParam.
We already offer relevance-based searching in vBulletin 3&4.