Memcached is a memory caching system. This will help to increase speed of your vbulletin or wordpress wesites. You can install it easily as follows,

Download the memcached server from http://memcached.org/

 wget -c  http://memcached.googlecode.com/files/memcached-1.4.15.tar.gz
 tar -xzf  memcached-1.4.15.tar.gz
 cd memcached-1.4.15/
 yum -y install libevent libevent-devel
 ./configure --prefix=/opt/memcached
 make
 make install

Now install init script . I wrote an init script for redhat and centos servers , you can use it to start/stop memcached.

 wget -c http://files.hostseo.com/conf/memcached.redhar.rc.txt
 mv -f memcached.redhar.rc.txt /etc/init.d/memcached
 chmod 750 /etc/init.d/memcached
 mkdir /var/run/memcached
 /etc/init.d/memcached start
 chkconfig --add memcached
 chkconfig memcached on

Install php-pecl memcache

Download the latest stable memcached from http://pecl.php.net/package/memcache

wget -c http://pecl.php.net/get/memcache-2.2.7.tgz
 tar -xzf  memcache-2.2.7.tgz
 cd memcache-2.2.7/
 phpize
 ./configure
 make
 make install

Now restart apache and create a phpinfo page and test whether the memcache options is showing or not. If it is there then you installed php module . If not you need to check your php.ini settings and enable memcache module

Vbulletin Configuration

Edit the Vb configuration file includes/config.php and uncomment the following lines

$config['Datastore']['class'] = 'vB_Datastore_Memcached';
$i = 0;
// First Server
$i++;
$config['Misc']['memcacheserver'][$i]           = '127.0.0.1';
$config['Misc']['memcacheport'][$i]                     = 11211;
$config['Misc']['memcachepersistent'][$i]       = true;
$config['Misc']['memcacheweight'][$i]           = 1;
$config['Misc']['memcachetimeout'][$i]          = 1;
$config['Misc']['memcacheretry_interval'][$i] = 15;

Now restart apache server . Your vbulletin now works with memcached. You can see the performance difference within 1 to 2 hours.

WordPress Configuration

You may just install the wordpress memcached plugin and configure it from wordpress admin panel