The MS-Sideshow Device app should be able to get this enabled for you. The link includes instructions and there is a video to walk you through it. The app uses PnP-X introduced in Windows Vista and enables an Android smart phone to be a SideShow device as long as both the PC and the smart phone are connected to the same Wi-Fi network.
Please be aware that users have reported mixed experiences and that the app has not been updated since Lollipop.
I thing you have a broken VGA cable. Try adjusting it and if it dosn't work buy a new cable. I reccomend the Insignia VGA cable from Best Buy (Not sponsored) https://www.amazon.com/Insignia-NS-PV12531-12-VGA-Cable/dp/B00OO4NVZO