cp /etc/yum.repos.d/CentOS-Base.repo /etc/yum.repos.d/CentOS-Base.repo.bak
vim /etc/yum.repos.d/CentOS-Base.repo
Comment out the mirrorlist directives. In each enabled section add baseurl=http://vault.centos.org/5.11/os/$basearch, baseurl=http://vault.centos.org/5.11/updates/$basearch, etc.
Reference: https://unix.stackexchange.com/questions/359902/yum-update-error-on-centos-5-6