I will need to keep my server os at 4.2 because that is the only one plesk supports so according to those 2 articles and staying at centos 4.2 will the "vault" files be the one to use and will they get security updates?
Vault files do NOT get security updates. vault.centos.org is where old versions go to die. 4.2+ security updates = 4.3, which from your description will not work with plesk.
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