The mobile style has to be imported
Go to AdminCP=> Styles and templates=> Download-upload styles
Under "Import Style XML File", enter
PHP Code:
./install/vbulletin-mobile-style.xml
Under "Title for uploaded style", give it a new 'mobile' name
Under "Parent style" choose an existing color scheme or leave it blank
Click "import"
Once imported, go to Settings=> Options=> Style & Language Settings
Select the new style under "Default Style for Modern Mobile Browsers"
It should now show up on your phone.