Completely uninstalling Astrill fixed the issue but this solution is not what I want. Astrill support answered (within 2 hours) saying the correct fix is to hold Ctrl and press the "Help" button on the Astrill application, and then choose "Uninstall LSP". This has fixed the issue without needing to un-install Astrill.

Visual Studio 2015 msvsmon.exe crashed when c++ debugging with x64