I'm using Visual Studio Community Edition 2015 with Unity. The UnityVS plugin is installed. The editor configured in Unity is Visual Studio 2015.
When I open a file in Unity it opens a new Visual Studio instance. I have tried via Asset -> Open C# project, double click file, click on Open button, etc.
How do I configure Unity (or Visual Studio) to keep a single instance?
You no londer need UnityVS for the latest Unity version. I can't remember which version UnityVS is no longer requred but I will suggest you try the following below.
Uninstall Visual Studio
Uninstall Unity
Restart Computer.
Install the latest Unity version from here.
Visual Studio should automatically install too. Make sure it is selected during Unity installation.
Don't install UnityVS or anyother plugin. It should work out of the box.