In order to conserve the limited bandwidth available .iso images are not downloadable from this Vault Machine.
Bandwidth usage for all other downloads are also limited, due to lack of nodes/bandwidth
The Following external Vault mirrors (not monitored by the CentOS Infra team !) provide direct downloads for all content, including isos:

USA:
http://archive.kernel.org/centos-vault/
rsync://archive.kernel.org::centos-vault/

Europe:
http://mirror.nsc.liu.se/centos-store/
rsync://mirror.nsc.liu.se::centos-store/