I’ve had to do this a couple of times now, so, mainly to save myself the effort next time, here’s how to get YUM up an running on a CentOS 5 VPS.
Download the following RPMs from a CentOS Mirror, making sure you match your architecture (eg: x86_64, or i386). If the mirror doesn’t contain the exact version listed below you should be OK grabbing a newer one 
- libxml2-2.6.26-2.1.2.8.el5_5.1.x86_64.rpm
- m2crypto-0.16-6.el5.8.x86_64.rpm
- python-elementtree-1.2.6-5.x86_64.rpm
- python-iniparse-0.2.3-4.el5.noarch.rpm
- python-sqlite-1.1.7-1.2.1.x86_64.rpm
- python-urlgrabber-3.1.0-6.el5.noarch.rpm
- rpm-python-4.4.2.3-22.el5.x86_64.rpm
- sqlite-3.3.6-5.x86_64.rpm
- yum-3.2.22-33.el5.centos.noarch.rpm
- yum-fastestmirror-1.1.16-14.el5.centos.1.noarch.rpm
- yum-metadata-parser-1.1.2-3.el5.centos.x86_64.rpm
Once you’ve downloaded them all, install them using the following command:
rpm -Uvh *.rpm
Have fun