Two minor corrections to my last post & one xtra bit of info ...
Correction #1: From what I can tell, the installer is identifying a Cent5 O/S & installing rpms that call for Cent6 Library objects. That should only happen if 1) The installer was compiled with a Cent6 version of libc.so.6 , or 2) If the installer in-correctly identifies the O/S.
Correction #2: I think that knowing what objects are inside Francesco's libstdc++.so.6 library will tell us a lot. (I'm betting his libstdc++.so.6 library doesn't include GLIBXX_3.4.11 either)
Xtra Info: I am able to get the rhel5 x32 client to install successfully if I 1) untar the baseclient & vulscan tarfiles, 2) rename the rhel5 rpms to overwrite the cent6 rpms, and 3) re-tar the baseclient & vulscan tarfiles. Unfortunately, running ldiscan still throws the library errors when executed.