First of all, VL 5.8 is no longer supported. That said, it is still quite useful and could be an educational introduction to VectorLinux.
The preferred method of installing and removing packages is using the package manager Gslapt. Look for it in the menu. You will need the root password. First thing to do is to click "Edit > Preferences > Sources" and make sure that the normal vectorlinux repositories are checked (packages, extra). I also like to include patches and testing. Next click on the "Update" button and let it finish. Then search for the programs you are looking for, using the search field.
If that doesn't do it, look for compatible Slackware packages. I believe 5.8 was based on Slack 11. Check that by looking in the file /etc/slackware-version. Some Limewire packages can be found here:http://www.linuxpackages.net/pkg_details.php?id=9949
Alternatives to Limewire: http://alternativeto.net/desktop/limewire/
To change defaults for opening file types, use Konqueror and right-click on the file in question. Choose "Open with > Other", pick the program and check the box for "Remember application association... ".