when a windows client access a share architecture variable (%a) is not correctly used. i'm not sure if i should file this as an enhacement. i've found some references: http://lists.samba.org/archive/samba/2010-July/157390.html but looking at code it seems that no work has been done.
Windows 7 is recodniced as VISTA. At the time %a has to be "guessed" we cannot tell the difference between VISTA and Win7 for sure so VISTA is that that you will have to expect if you connect with Windows 7, too.