In order to conserve the limited bandwidth available .iso images are not downloadable from this Vault Machine.
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/