Before doing any restore, select the "Sent" folder, then View, Current
View and make sure that Show All Messages is checked and nothing else.
Your missing messages might just be hidden.
Not having Vista, I'm not familiar with the capabilities of that
program. Does it allow you to restore to an alternate location?
If so, restore the
%userprofile%\Local Settings\Application Data\Microsoft\Windows Live
Mail
folder to a new folder. Then in WLM you can use File, Import, Messages,
From WLM and browse to the restore location and select the folders that
you want to import. After import, look for them under Storage Folders,
Imported Folders. You can then move the messages to the folder that
they need to be in.