There's a vbulletin plugin that auto-sets the mobile style for mobile browsers. I'm sure they're using this, but haven't tried yet on my phone.
What I've done on my own forum is to set m.xxxxxxx.com to use the mobile style by default. This helps the people whose phones aren't recognized by the plugin. They can just set that bookmark.
The drop-down works too, but most people won't think to look down there.