Try increasing the memory limit in php.ini. The link to your phpinfo states that it is 32M. Increase it to something much larger at first, like 400M or 800M if your server has the RAM available. Also, reduce your xcache counts to 1, for both the primary and var cache. Make your var cache size 4M.
Restart your webserver and see what happens. 503 errors are common with temporary overload of server resources. Something is getting in the way.
Some VPS technologies don't play well with XCache, due to the way they share memory. If you continue to have problems, I'd open a ticket with vbulletin and contact your host.
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