Manual:$wgVectorUseSimpleSearch

Details
Controls search form behavior for Vector skin only:


 *   = use one icon search button

Since MediaWiki 1.23, if JavaScript is enabled in the browser, only the Go button will be shown with $wgVectorUseSimpleSearch = false;. If JavaScript is not enabled, then both buttons will be visible.
 *   = use separate "Go" and "Search" buttons.