Vista Ultimate on 2003 Domain

I am having an interesting problem when it comes to modifying file/folder
permissions on networked drives from the Vista computer. When I access the
security tab I can view all the current permissions with no problem, I can
even go in a take control of files/folders. When I try to add a specific
user account to a file/folder that already has 3 user accounts specified, the
new user account replaces one of the others. The only way I have been able
to set the permissions is to not use the Vista machine, but to log in to the
server or an XP Pro machine and make the changes.

Any thoughts?