Windows Vista & Offline Files

Hi,

I have a client with Vista Business, Connected to SBS 2003. Offline
files are enabled and working. However, when a user opens a document
on the server (while online) and then makes changes, these changes
happen to the cached file and not the server file. They are not
transferred until a manual sync is performed.
If the Vista client has a file open and another (XP) machine opens the
file, there is no warning the file is open, and vice versa.
I need to resolve this as they have had some issues where files have
been edited by 2x clients and the vista one eventually dumps it's copy
over the top during a sync.
Anyone got any ideas?

Thanks.