CentOS server seems to deny access to your client (yum)! Crazy, I can’t reproduce it. The following command works as expected on my host, by retrieving an XML document:
curl -H 'Cache-Control: no-cache' -v -A 'urlgrabber/3.9.1 yum/3.2.29' http://vault.centos.org/6.7/os/x86_64/repodata/repomd.xml
Could you paste the output of the command above? Are you behind a web proxy?
BTW: I suggest to update your system to v6.8!