The old mobile site was based on the WiForums WAP platform. It was a web application that connected directly to the database and had its own method for parsing the data. It was optimized for early smartphones. The new skin (I assume it is a skin, that is) is likely a direct add-on for vBulletin. This is likely auto-selected with a browser agent detection script for mobile devices. Clicking View Full Site likely changes your skin selection back to the default. This is the method we used to use on my vBulletin website.