Here's what Google says (source):It'd be interesting to see why the revenue for https sites is low.
HTTPS-enabled sites require that all content on the page, including the ads, be SSL-compliant. As such, AdSense will remove all non-SSL compliant ads from competing in the auction on these pages. If you do decide to convert your HTTP site to HTTPS, please be aware that because we remove non-SSL compliant ads from the auction, thereby reducing auction pressure, ads on your HTTPS pages might earn less than those on your HTTP pages.
Except that Google is going to use https as a ranking signal, meaning that the content itself will see less traffic if we don't serve it on https pages.I'm not sure how vBulletin does what it does but perhaps it can be setup to secure the login itself (ie credentials aren't passed in the clear) while leaving the rest of the site as standard http (ie if you're not logging in and passing sensitive data you don't get secured). After-all, the content itself is publicly accessible and visible, its only the login portal that is sensitive.
Thanks as always!I'll be hitting my 2000th post soon so I'll be sure to top up my site donation to aide the cause