I'm not quite sure, it's been sometime since I left Fedora, but my first guess, for libxml would be:
# yum search libxml
If yum finds anything, try
# yum info <package>
to see if it is what you're expecting. Probably some package named "libXML" or so...
For VLC, I remember to just
# yum -y install vlc
and it was all set, just the MP3 and ACC codecs that weren't installed automatically because of some licence issues.
Note: Must be root in order to install packages. The "-y" flag on the yum command line says "yes" to all questions, be careful (and sorry if you already knew that).