Hm... So in the VaultWiki code, lightbox.js IS added to a list of possible Javascript load-on-demand files, and that file list is generally output to the browser. However, (text-search confirmed) there is no code in VaultWiki that actually triggers that file to load-on-demand. So I am not sure where you are seeing this file fail to load, or what would make it try to load. I cannot find an example on your site.
I also don't think it is related to the AJAX preview issue since AJAX previews are not working even on pages with no lightbox error (as I mentioned I can't even find a lightbox error). From what I can tell, it just seems that some code is not running on your site. It is possible this is due to timing issues or small code changes introduced by running mod_pagespeed. I am not sure that mod really provides much benefit on your Javascript under XenForo, since both XenForo and VaultWiki provide minified scripts anyway, and the scripts are generally already as deferred as possible in their design. From what I can see, you might be saving network latency on 1 file in total, which may not be worth the added execution time and lower reliability overall of using mod_pagespeed in this way.