After suffering with this for months, I starting doing some searches today and ended up here. This is the top hit under discussions.apple.com on google when searching "itunes wifi sync ethernet".
My Vista machine is on my wired gigabit ethernet and I've always found wifi sync to my former phone (iphone 4s) and current phone (5s) to be flaky at best. This has been the case across multiple versions of iTunes as well. The problem I have is that the device usually does not show up in iTunes. I have found that if I just wait anywhere from 3-10 minutes, it will in fact show up and then a sync is possible, although sometimes that sync is then slow and sometimes it does not complete. When it does not complete successfully, I've never seen any kind of error message. I discover the failure by the mismatch between what is on the PC and what is on the device, later, often long after the sync "finished" and I am nowhere near the PC. Sometimes it works "well", that is it does not seem inordinately slow and does complete successfully.
I did find a couple of things in doing research that are relevant:
1. Apple indicates that syncing PC to iOS device when the PC is on a wired ethernet is not supported: Get help with iTunes Wi-Fi syncing - Apple Support "Check for network configuration changes that may interfere with syncing" ... "Connecting the computer over Ethernet"
2. Restarting the "Apple Mobile Device Service" may help. This should be done while iTunes is not running.
3. Rebooting the PC may help.
4. Doing a winsock reset may help: "netsh winsock reset”
I found some information indicating that the iOS device must be connected to a power supply to sync. I found that not to be true-- having the power connected changes nothing as far as I can tell-- the device appears in iTunes at the same frequency, with the same delay and the sync works just as well/poorly with or without power connected.
