Seems this was happening on my setup. Sometimes VPN would sync, sometimes not. The VPN always connects okay to the LAN. I uninstalled the network sync service and all seems to be well for the time being. I haven't checked deeply enough, but I wonder if there is a predefined connection hierarchy?
I'm a little confused about what you see happening. Are you getting an error message when attempting to sync when you connect in a certain order, if so, when in the process are you getting that error message?
If you could provide a few more details, I'll do my best to provide you with a solution.