It is possible to share files using hard links. vunify was created to merge common package using hard link so they use the same disk space. It works like this
/usr/lib/vserver/vunify refserver serv1 serv2 serv3 -- pkg1 pkg2 pkg3 ...
refserver is the reference server. serv1 to serv3 are other vservers and all vservers have a copy of the same RPM packages pkg1, pkg2. The utility will extract the list of file own by the various packages. Configuration files are omitted from the list. Then it will walk the vservers and erase the files and setup a hard link pointing to the file in refserver. Once done, it will turn the file immutable bit. This bit prevent the vservers from modifying the file. Even root can't do it. You will end up saving a log of disk space. You may gain some performance as well since the various shared object will be loaded only once in memory for all vservers. This could be noticeable if you have many vservers.
Experiment have shown that unifying the packages bash, glibc, perl and binutils save 60 megabytes per vserver. Given those package seldom change, this is a nice saving.
vunify also feature a --undo option to un-unify some package. It turns off the immutable bit and place a copy of the files in each vservers. This can be useful if you intend to update one package.