@snakes - yes, I was using vbulletin's detection, which doesn't 'force' a style but it does pre-select the lightweight style for mobile devices, and allows changing to other styles after initial detection. I turned this off and the style no longer sticks (from my Nexus) as you reported - neither does vbulletin's style so I can only assume the issue is with vbulletin. It does 'stick' from my desktop browser.
I've questioned this behaviour on
vBulletin.com