Your current code only check whether Wifi enabled or not on the device, which will always return false if you run in emulator. Because there is no interface to set Wifi enabled in WP7 emulator. You need to use IsNetworkAvailable
property instead, to check if any internet connection available. See this MSDN link for reference. So your code should be as follow :
Dim _isNetworkAvailable = Microsoft.Phone.Net.NetworkInformation.DeviceNetworkInformation.IsNetworkAvailable