I have one of those. It did get an update a few months back, but nothing changed in the OS details in About Phone, not even the security details date.
Vodafone were marketing it quite heavily, so you'd sort of expect it to have been popular enough to get an update, but seemingly not. That is one of the downsides of Android - so many people have a hand in the OS: Google, the manufacturer and the network if the device is branded. Some people stick up SIM-free for that reason and Sony are generally quite good about updating older devices.
The Sony website does have an option to enter your IMEI and check what the latest OS version is, but if you've checked for updates and read the About details, you already know. It won't tell you what may or may not be coming, sadly.
I have another device that's running Nougat and, for most purposes, you can't tell the difference between that and Marshmallow.
