My entire Vbulletin 4 forum is running though SSL/HTTPS, and it runs perfectly fine. I even installed some Optimized Addons to make the pages load faster.
I havent had any problems at all with running my forum on HTTPS.
Yeah, if someone posts an image from a site using HTTP with the Image BBCode, there will be a tiny little Yellow sign (Chrome) in your browser on top of the Padlock. But no one sees a Security Warning or anything like that. Honestly, you wouldn't even know about the Tiny Yellow Sign if you weren't looking for it, because it really isnt a big deal. If it was a big deal, the user will see a Big Security Warning before he or she enters the page.
But like I said, running SSL with vBulletin is fine. It runs really, really well. I have even setup my web.config (Windows Server 2008R2) to redirect users to HTTPS. So if they type in "mysite.com" in their address bar, it would redirect them to
https://mysite.com.
I setup a Test Thread on my site, and I posted an image from tinypic.com that uses HTTP and not HTTPS.
Check it out for yourself:
https://thetechgenius.net/threads/4-Test-Thread