vBulletin Manual - Enabling the Mobile Style

If you do wish to use the mobile style, you can enable it following these steps:

Admin CP -> Settings -> Options -> Style & Language Settings -> Default Style for Old Mobile Browsers. Select 'None' from the Dropdown menu.
Admin CP -> Settings -> Options -> Style & Language Settings -> Default Style for Modern Mobile Browsers. Set 'Default Mobile Style' from the dropdown menu.
Admin CP -> Styles & Templates -> Style Manager. Edit any style in the mobile style list and set "Allow User Selection" to Yes.

Note:

Note: Not all Old Mobile Browsers will be able to use the Default Mobile Style. For the purposes of these older browsers, you may be able to find a style available at www.vbulletin.org.