My experience is that with nginx and with no addons at all, the forums almost loads instantly. But this also depends on the database size, the server and tons of options management with vB4 that changes the loading speed, eg:
http://www.vbulletin.com/forum/forum...ms-vbulletin-4
While from 2010, quite a few options are very helpful.
I've read on a lot of sites that memcached is only helpful when it is running with multiple servers, rather than one. With only one, xcache would be better. But in general I skip xcache/memcache and go with filecache when the server is running on SSD.
My 2 cents.