Well i can see at first sight 2 series of potential problems that could slow down server.
1. vbgallery issues.
Vbgallery 2.5 has some problems with big boards. (when I say big, I do not meen the amount of users, but the amount of pictures & users)
These issues have been removed in vbgallery 3.0 but cannot be in 2.5 as it would require rewriting lots of code.
These issues would slow down gallery pages and take server load to increadible peeks.
These issues were: A. filmstrip in showimage
. Turn off filmstrip (set to 0)
Filmstrip makes an array of ALL images in category .. if you have like 50.000 images per category.. you will get timeouts. B. Category Jump. Switch off category jump
in ACP. (its ok if you have like 10 categories, but some boards have like 300 categories and the catjump is generated every pageload. Second series of problems..
I have no experience with ubuntu, but when you say you are running PHP: 5.2.4-2ubuntu5.3 is that php 5.2.4 or a compatible version of 5.3?
Just wondering, because on php 5.3 you are bound to have a series of issues with vbulletin 3.7.3 and vbgallery 2.5
why? because php would issue a great series of deprecated warnings for both, vb
3.7 and vbgallery, even if you suppress those warnings in php.ini, that would still noticably slow down your system. But with 5.24 you should NOT have those problems.
PS try at first to turn of category jump and filmstrip.