Man, why are you running an insanely weird set of insanely outdated php/MySQL/apache.
Upgrade apache to a minimum of 2.2.31 but preferably 2.4.18, though that is a major upgrade so use caution (the 2.4.x branch)
Upgrade MySQL to latest 5.5 or 5.6 branch. IIRC there are some weird issues with MySQL 5.7 and vB4, not sure if they've been patched yet. Pro Tip: backup all of your databases prior to upgrading.
Upgrade PHP to a minimum of PHP 5.5.31, though 5.6 would be way better. 5.3.3 is a notoriously buggy and bad build of PHP.
Unless you can't for some reason you should upgrade to the least vBulletin 4.2.x build as well, there isn't a great reason not to.
After you do all that, I would really consider checking in with George on vBulletin.com assuming he is still posting server config help.
As one last recommendation, if you're sending a tiny amount of emails per month (less than 10k) and you're not taking advantage of mail services on your server, you could free up a lot of ram and system resources.
https://sendgrid.com/pricing SendGrid Free or Lite area both stupid cheap. MailChimp also has a cheap plan:
http://mailchimp.com/pricing/entrepreneur/ lastly Amazon has SES with 60k messages In its free tier, but there may be other requirements:
https://aws.amazon.com/ses/
Really it'd be a lot better to outsource your email anyway. If you need your own email GoogleApps / Office 365 business might make a lot more sense (esp for 1-2 accounts).