Looking at my post on Vista Windows Mobile Device Center (WMDC) there are a huge number of comments/posts relating to technical issues trying to synchronise with Vista.
To hopefully help out 99.999% of people I wanted to post the key steps to getting Windows Mobile Device Center up and running...
To get WMDC installed you need to:
1) First connect your Windows Mobile device to your Vista PC. Check it has been recognised and installed the necessary drivers for it.
2) Then go to Windows Update and search for new updates. You should see the Windows Mobile Device Center - simply install that update and also should work.
If it doesn't work....
For most people the process above works fine. There are a number of things to check if you are in the minority that it doesn't work for....
1) Leave your device connected whilst you search Windows Update.
2) Try switching the device back to a Serial connection from RNDIS. Just like Activesync - firewalls and various VPN software can cause issues with your device connecting to Vista because it uses RNDIS (which makes the connection look like a network connection). To switch your device back to using Serial do the following:
a) Go to Start... Settings... Connections... Find USB to PC
b) Clear the checkbox that says Enable Advanced Networking Functionality. This switches the device back to serial mode.
This resolves about 99.999% of problems. You need a device that has AKU 2.3 or greater to be able to do this. (to check go to Start...Settings..About and check the Version number - the build should be greater than xxxxx.2.3.x)
If you still have problems outside of the troubleshooting above I'd suggest disabling any VPN software, Firewall software and bandwidth optimisers as these are normally impacting the sync capability.