Be sure that you can ping vault.centos.org.
Then edit /etc/yum.repos.d/CentOS-Base.repo
Comment out mirrorlist and uncomment baseurl
Change all
baseurl=http://mirror.centos.org/centos/$releasever/centosplus/$basearch/
to
baseurl=http://vault.centos.org/$releasever/centosplus/$basearch/
And enjoy your yum update!!! 😉