Another thing: while vmstat is good for immediate problem solving, it's useful to setup permanent monitoring, which gives you a picture of your system over time. An example is http://munin-monitoring.org/ (there are many others). This will help correlating system changes with performance changes and doesn't cost you many resources. Most systems will out of the box monitor things like CPU, memory and I/O on Linux, and many will offer plugins for specific applications like vBulletin and MySQL, offering you a complete picture.
I can't work anymore without something like that...
I can't work anymore without something like that...