I got this:

[root@localhost ~]# yum install vsftpd

Loaded plugins: fastestmirror
Loading mirror speeds from cached hostfile
* base: ftp.cuhk.edu.hk
* updates: ftp.cuhk.edu.hk
* addons: ftp.cuhk.edu.hk
* extras: ftp.cuhk.edu.hk
http://ftp.cuhk.edu.hk/pub/Linux/cen...ta/repomd.xml: [Errno 4] IOError: <urlopen error (-3, 'Temporary failure in name resolution')>
Trying other mirror.
http://mirror.vpshosting.com.hk/pub/...ta/repomd.xml: [Errno 4] IOError: <urlopen error (-3, 'Temporary failure in name resolution')>
Trying other mirror.
http://ftp.twaren.net/Linux/CentOS/5...ta/repomd.xml: [Errno 4] IOError: <urlopen error (-3, 'Temporary failure in name resolution')>
Trying other mirror.
http://centos.ustc.edu.cn/centos/5.3...ta/repomd.xml: [Errno 4] IOError: <urlopen error (-3, 'Temporary failure in name resolution')>
Trying other mirror.
http://centos.cs.nctu.edu.tw/5.3/os/...ta/repomd.xml: [Errno 4] IOError: <urlopen error (-3, 'Temporary failure in name resolution')>
Trying other mirror.
http://ftp.cse.yzu.edu.tw/pub/CentOS...ta/repomd.xml: [Errno 4] IOError: <urlopen error (-3, 'Temporary failure in name resolution')>
Trying other mirror.
http://ftp.isu.edu.tw/Linux/CentOS/5...ta/repomd.xml: [Errno 4] IOError: <urlopen error (-3, 'Temporary failure in name resolution')>
Trying other mirror.
http://ftp.tcc.edu.tw/Linux/CentOS/5...ta/repomd.xml: [Errno 4] IOError: <urlopen error (-3, 'Temporary failure in name resolution')>
Trying other mirror.
http://ftp.cs.pu.edu.tw/Linux/CentOS...ta/repomd.xml: [Errno 4] IOError: <urlopen error (-3, 'Temporary failure in name resolution')>
Trying other mirror.
ftp://ftp.chu.edu.tw/Linux/CentOS/5....ta/repomd.xml: [Errno 4] IOError: <urlopen error (-3, 'Temporary failure in name resolution')>
Trying other mirror.
Error: Cannot retrieve repository metadata (repomd.xml) for repository: base. Please verify its path and try again

It looks like that mirror is down. run 'yum clean all' and try again. That should get you a new "fastest" mirror to pull from.

still cannot. I try other file and other site have same result

Then you need to verify your /etc/resolv.conf is pointing to the correct named servers. You will need to consult with your network admins or ISP for the correct ones.