Sounds like the plugin/plugins you want aren't enabled.
You can enable a plugin by creating a symlink to /usr/share/munin/plugins/plugin_name in /etc/munin/plugins/ on each of the servers running munin-node.
E.g. to enable apache_accesses, apache_processes and apache_volume, run (as root):
ln -s /usr/share/munin/plugins/apache_accesses /etc/munin/plugins/apache_accesses
ln -s /usr/share/munin/plugins/apache_processes /etc/munin/plugins/apache_processes
ln -s /usr/share/munin/plugins/apache_volume /etc/munin/plugins/apache_volume
/etc/init.d/munin-node restart so that munin-node sees the new plugins.
If it still won't work, then maybe you don't have the Apache server-status page enabled on those particular VPSes (all three munin apache plugins require it).
I prefer to use nginx so I can't give you specific instructions from experience, but this page has a nice-looking guide to setting up mod_status on Ubuntu. Unless you've got a good reason, you should restrict it to localhost like that page says (it can be considered a security vulnerability to leave it open to the world since it shows information about your server, your visitors' IP addresses and the URLs they're accessing.)