I tried the zend opcode cache on a new server just to see what it does to Vbulletin but I am really not impressed. It might just be my own lack of understanding/knowledge but the end result was that APC is doing a lot better for me.
Also this is with apc.stat turned on as I have some wordpress running on the same server. I am not sure how would xenforo behave with apc.stat turned off but if it doesnt break then that would reduce the page load times even furthur.
Zend opcache is not a replacement for APC. You would need something like a memcached to go with zend opcode cache. APC does a lot more then what zend opcode cache does because zend opcode is purely an opcode cache and nothing beyond that.
Zend opcode will use a lot more memory as compared to APC to achieve similar performance and also zend wont cache data which APC does for you.
Also zend will restart again and again after a certain time which APC doesnt have to. Zend opcode does not reuse the memory slots and keeps burning new land to cultivate Its still in its nomadic days. APC does a lot better job with very little. When zend opcode refreshes itself once the memory has exausted there are a few moments when your site's performance would take a hit. APC wont do that if you have piped logging turned on and dont need apache to restart every two hours for bandwidth logs (thats default for cpanel installs).
I would appreciate if someone could correct me if I am wrong.