Windows Mobile Device Centre has known synchronisation problems where it stops recognising a PDA.
After struggling with this problem for quite some time I found a work around that solves this issue.
To pull this off you will need a second account on your Vista machine.
Click start and >> switch user. Login to your alternate windows profile.
Run WMDC. You will notice that it works on this profile. Once it finds your device, log off switch back to your main profile.
In my case this allowed WMDC to continue recognising my device and synchronisation worked fine.
This is short term solution until Microsoft can provide a fix for this problem.
A permanent solution would be to migrate all your documents and settings to the alternate profile and continue using that instead.