Visual Studio 2012 and newer
By default, Visual Assist opens files in the preview tab of the IDE, if the appropriate option is enabled in the options dialog of the IDE.
Visual Assist will not use the preview tab if the appropriate option is disabled, i.e. if the IDE itself does not use the preview tab. If you wish for the IDE to continue its use of the preview tab, yet prefer Visual Assist ignore the tab altogether, set the following registry DWORD value:
HKCU\Software\Whole Tomato\Visual Assist X\<IDE spec>\UsePreviewTab = 00
Restore the value to 01 for default behavior, i.e. to make Visual Assist respect the preview tab.